I'm assuming this has to do with some sort of DHCP clash for IPv6 since my modem does not allow disabling it, but this behavior is new of recent Pi-hole versions. It even causes issues for a device with manual address assignment, which is insane.
For the Xiaomi devices part, there's not anything much interesting to look at: when connected to an AP in the network, after around 8 seconds they disconnect, and then they reconnect once again. This goes from Xiaomi phones, to a Mi Stick (pretty much anything Xiaomi running Android).
About the logs, what sort would you like me to provide?
DHCP is a strictly IPv4.
The IPv6 equivalent is DHCPv6 - different ports, different protocol, and most notably, not supported by Android at all.
Do you need Pi-hole's IPv6 support at all?
Pi-hole's IPv6 support would allow clients to request an IPv6 DNS server address via Stateless DHCPv6 (which an Android client would never do), and it would advertise IPv6 DNS server options via RA, to be picked up by clients using SLAAC.
In most cases, this would not be necessary, as it would be your router that absolutely MUST be configured to advertise and offer Pi-hole's IPv6 DNS server address anyway, or not to advertise any IPv6 DNS server at all.
A router that cannot be configured either way will likely advertise its own IPv6 as DNS server, allowing clients to completely by-pass Pi-hole via that IPv6 at their own discretion.
Adding Pi-hole's IPv6 support into that mix would have no effect on your router. It would just lower the probability of IPv6 clients using your router instead of Pi-hole's IPv6 or IPv4 address.