About a million years ago, back in 2007/2008 that is, there was this small company called Hexago that did R&D in IPv6 networking, they were behind the Frenet6 project and created the networking stack and the TSP client that would let you tunnel a /56 IPv6 network over a dynamic IPv4 connection.

One the projects was a tiny hardware router, I honestly forget who made it, but Hexago would buy them, then we would flash each one with WRT+TSP client custom image, the idea was you plug this in your network and you have IPv6 connection in your network without doing any magic configuration.

It worked well until we lost finding.

So yeah, OpenWRT is old and not just for Linksys routers :)

DHCP?

Your old host registered an fqdn and did mnt remove it, then the new host registered the same ip to a different fqdn.

It happens

I have no idea what you are using for dhcp/dns but start by looking there