Something similar happened to me with MT6000 firmware 4.6.2
Tried my best to narrow it down but the best I could get to was that I was assigning too many IPs at once.
Unlike the OP, I was not using ipv6.
Everything started with a "Privacy Warning" in my phone:
"This network is blocking encrypted DNS traffic.
The names of websites and other servers your device accesses on this network may be monitored and recorded by other devices on this network."
I assumed it was the AdGuard Home, so I disabled it. Same message but I started to notice devices in the network to disconnect. Generally I have about 22 devices and it went down to 12.
After enabling/disabling AdGuard a few times I could notice my Address Reservation list was back to the one from like a month ago (about 5 devices reserved). At this point with AdGuard disabled, I proceeded to rebuild my Address Reservation list, and slowly I kept losing devices again.
I tried deleting the new items and the devices reconnected.
Now my network is happy but my ocd kicks in from time to time as I really wanted to organize all my IPs.
I am not sure if this could be relating to naming devices with special characters like ' " ( [.
Seems to work when just naming the device but not when assigning an IP.
Another thing I thought about was that the DHCP feature might not be ok with reassigning an IP while the device is connected. At this point I was already 3 hours in of troubleshooting so did not test to disconnect the devices completely ( I did test rebooting but did not work), then reconnect.
Hope this could narrow down the investigation a bit for the GLiNet folks.
Thanks!