IPv6 routing does not work on LAN, but does work from router

Hello, I have installed OpenWRT r25239-a3a33f02ce on the Linksys E8450. This has been working as desired for about a month, but this morning at about 7:15am local time on Mar 23rd (according to my network monitoring), IPv6 stopped working for all devices on all VLANs.

The router is able to reach IPv6 endpoints:

root@yumi:~# wget -O/dev/null -6 https://example.com
Downloading 'https://example.com'
Connecting to 2606:2800:220:1:248:1893:25c8:1946:443
Writing to '/dev/null'
/dev/null            100% |*******************************|  1256   0:00:00 ETA
Download completed (1256 bytes)

However, devices on the network are not able to do so:

{raxod502@kelsier} ~ % curl -v -6 https://example.com
*   Trying 2606:2800:220:1:248:1893:25c8:1946:443...
* connect to 2606:2800:220:1:248:1893:25c8:1946 port 443 failed: No route to host
* Failed to connect to example.com port 443 after 3078 ms: No route to host
* Closing connection 0
curl: (7) Failed to connect to example.com port 443 after 3078 ms: No route to host

The following IPv6 routing table is defined for a device on the local network:

{raxod502@kelsier} ~ % ip -6 route
::1 dev lo proto kernel metric 256 pref medium
2607:f598:b15a:5c1::/64 dev wlp166s0 proto ra metric 600 pref medium
2607:f598:b15a:5c0::/60 via fe80::8069:1aff:fe23:8d77 dev wlp166s0 proto ra metric 600 pref medium
fd11:2198:bc83:1::/64 dev wlp166s0 proto ra metric 600 pref medium
fd11:2198:bc83::/48 via fe80::8069:1aff:fe23:8d77 dev wlp166s0 proto ra metric 600 pref medium
fe80::/64 dev wlp166s0 proto kernel metric 1024 pref medium
default via fe80::8069:1aff:fe23:8d77 dev wlp166s0 proto ra metric 20600 pref medium

I configured an internal network and guest network:

config interface 'lan'
	option device 'br-lan'
	option proto 'static'
	list ipaddr '192.168.1.1/24'
	list dns '9.9.9.9'
	list dns '149.112.112.112'
	list dns '2620:fe::fe'
	list dns '2620:fe::9'
	option ip6assign '64'
	option delegate '0'
	option ip6hint '0'

config interface 'guest'
	option proto 'static'
	option device 'br-guest'
	list ipaddr '192.168.3.1/24'
	list dns '9.9.9.9'
	list dns '149.112.112.112'
	list dns '2620:fe::fe'
	list dns '2620:fe::9'
	option delegate '0'
	option ip6assign '64'
	option ip6hint '1'

The following information is shown under the IPv6 Upstream section in LuCI:

Protocol: DHCPv6 client
Prefix Delegated: 2607:f598:b15a:5c0::/60
Address: 2607:f598:b150:252:0:a:0:1f/128
Address: 2607:f598:b150:252:8269:1aff:fe23:8d75/64
Gateway: fe80::227:e3ff:fe78:93db
DNS 1: 2607:f598:0:1::2
DNS 2: 2607:f598:0:1::3
DNS 3: 2001:4860:4860::8888
Connected: 27d 18h 47m 18s
Device: Switch port: "wan"
MAC address: 80:69:1A:23:8D:75

Based on this my understanding is that my ISP has delegated a /60 prefix to me. My intent is to use one /64 sub-block for the internal network, and another /64 sub-block to the guest network, which would utilize one-eighth of the available address space. This is why I set unique ip6hint values for each network.

IPv4 works correctly on all devices on all VLANs. Before this morning, IPv6 also worked correctly on all devices on all VLANs; now, it doesn't work on any of them, but IPv4 is still fine. I am not aware of any configuration changes that were applied this morning; in fact, I was asleep at the time.

I uploaded the system log, which covers the time at which routing stopped working: https://gist.github.com/raxod502/b761df9c36e9bf9e39e27b284e65ad2d

What should I look into or learn about in order to understand what could be wrong? Is there any other diagnostic or configuration information I should provide here? (I checked the wiki, but couldn't find any information on suggested information to include in a forum post.)

Update:

At about 3:05am on Mar 24th local time (according to my network monitoring), with no configuration changes on my part, IPv6 routing started working again for all devices on all VLANs. Here is the system log from this morning: https://gist.github.com/raxod502/c9136a106a807e35fa8ba517394fbaf7

I still have no idea what is going on. The fact that the system recovered on its own suggests to me that perhaps rebooting the router could have remediated the issue, but I'd prefer to understand why it became broken in the first place, especially given that IPv6 routing clearly worked on the router itself, and only appeared to be an issue with OpenWRT network configuration.