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.