Even I go with one of the better providers in my country even a "normal" reboot from time to time ends up for me to wait like 20 to 30 minutes till I get a pppoe response and a new dhcp lease and dhcpv6 PD 
I doubt that such an old and low feature gets broken suddenly. Even if someone did major rewrites I would expect lots of unit tests which should highlight bugs long before code gets committed.
Did you checked the open issues or did you opened a new one?
@systemcrash @_bernd
I reset the settings of my custom build to default just to check but no IPv6.
I tested the latest available snapshot build from firmware-selector with default packages and first boot settings but still no IPv6.
So I assume there is an issue in a recent commit.
Complete the procedure I highlighted to determine the problem
./scripts/update_git_source_package.sh ./package/network/services/odhcpd/Makefile 053e6c4b386348834e35e5fefc921fadc0e64025
Cloning into bare repository '/tmp/tmp.nIvUkKLXrK'...
remote: Enumerating objects: 3300, done.
remote: Counting objects: 100% (3300/3300), done.
remote: Compressing objects: 100% (2393/2393), done.
remote: Total 3300 (delta 2544), reused 1197 (delta 900), pack-reused 0
Receiving objects: 100% (3300/3300), 828.57 KiB | 2.31 MiB/s, done.
Resolving deltas: 100% (2544/2544), done.
fatal: Invalid revision range fc27940fe9939f99aeb988d021c7edfa54460123..053e6c4b386348834e35e5fefc921fadc0e64025
Unable to determine changes from commit 'fc27940fe9939f99aeb988d021c7edfa54460123' to '053e6c4b386348834e35e5fefc921fadc0e64025'.
Update
Thanks for your help.
I've disabled WAN6 interface for the last 12 hours and just a few minutes ago I re-enabled it and it simply got all IPv6 attributes. Weird ISP stuff.
Just to close this case I add some more information.
I updated two other routers that use the same ISP and the same IPv6 glitch appeared in both cases. I had to disconnect WAN6 interface for 24 hours and then when I restarted WAN6 interface IPv6 worked OK.
Did you try the routine as I suggested?
No, unfortunately I couldn't try it, sorry. It was a remote upgrade and I just forgot to try it.
After the latest update from today I'm unable to get IPv6 again.
The error messages
user.notice: ucitrack: Setting up /etc/config/odhcpd reload dependency on /etc/config/dhcp
daemon.err: odhcp6c[6885]: Failed to send RS (Address not available)
daemon.err: odhcp6c[6885]: wan: error connecting LLA socket
daemon.info: dnsmasq[1]: read /tmp/hosts/odhcpd.hosts.lan - 0 names
daemon.warn: odhcpd[4037]: No default route present, setting ra_lifetime to 0!
daemon.err: odhcp6c[6885]: Failed to send SOLICIT message to ff02::1:2 (Address not available)
daemon.warn: odhcpd[4037]: No default route present, setting ra_lifetime to 0!
daemon.warn: odhcpd[4037]: No default route present, setting ra_lifetime to 0!
daemon.info: dnsmasq[1]: read /tmp/hosts/odhcpd.hosts.lan - 0 names
daemon.info: dnsmasq[1]: read /tmp/hosts/odhcpd.hosts.lan - 0 names
daemon.warn: odhcpd[4037]: No default route present, setting ra_lifetime to 0!
I'm at this commit.
tcpdump: listening on wan, link-type EN10MB (Ethernet), snapshot length 262144 bytes
03:52:42.727380 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0xdfa1!] dhcp6 solicit (xid=4a8ec9 (elapsed-time 65535) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
.....
03:53:27.906867 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0x6deb!] dhcp6 solicit (xid=bf000b (elapsed-time 0) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
.....
03:53:28.179889 IP6 (class 0xc0, hlim 255, next-header ICMPv6 (58) payload length: 24) fe80::a2f3:e4ff:fe81:fe30 > fe80::642:1aff:fee4:d11d: [icmp6 sum ok] ICMP6, router advertisement, length 24
hop limit 64, Flags [managed], pref medium, router lifetime 4500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): 00:00:5e:00:01:13
0x0000: 6c00 0000 0018 3aff fe80 0000 0000 0000 l.....:.........
0x0010: a2f3 e4ff fe81 fe30 fe80 0000 0000 0000 .......0........
0x0020: 0642 1aff fee4 d11d 8600 5498 4080 1194 .B........T.@...
0x0030: 0000 0000 0000 0000 0101 0000 5e00 0113 ............^...
03:53:28.921418 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0x6d86!] dhcp6 solicit (xid=bf000b (elapsed-time 101) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
.....
03:53:30.871595 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0x6cc3!] dhcp6 solicit (xid=bf000b (elapsed-time 296) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
.....
03:53:34.634756 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0x6b4b!] dhcp6 solicit (xid=bf000b (elapsed-time 672) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
.....
03:53:41.959449 IP6 (flowlabel 0xc7173, hlim 1, next-header UDP (17) payload length: 109) fe80::642:1aff:fee4:d11d.546 > ff02::1:2.547: [bad udp cksum 0xef48 -> 0x686e!] dhcp6 solicit (xid=bf000b (elapsed-time 1405) (option-request SIP-servers-domain SIP-servers-address DNS-server DNS-search-list SNTP-servers NTP-server opt_67 opt_103 opt_82) (client-ID uuid b390d93988b94991af80b079510b9821) (reconfigure-accept) (Client-FQDN) (IA_NA IAID:6 T1:0 T2:0) (IA_PD IAID:1 T1:0 T2:0))
@systemcrash
Sorry to bother you again about this. I don't understand but just 10 minutes ago the router got the IPv6 from my ISP. Athough I restarted the ISP device yesterday it couldn't get IPv6 address for at least 12 hours.
It was the same situation as I reported previously.
This time before the sysupgrade I had those set.
Oh! What did you have set?
I did this.
Is your DUID stable? That client-ID uuid looks unusual.
I recall you have a weird upstream provider where multiple seem to respond. But your capture log shows you don’t even see a response?