[x2tyfi]

Probably a routing issue. Shot in the dark would be that one of these routers is NATing traffic, and the other router doesn’t have a route to that NAT’d range.

[tonyarkles]

Other shot in the dark, misconfigured bridging or similar where ARP isn’t getting forwarded and rewritten quite right

[bombcar]

Or look for something mangling SRV records, Minecraft seems to use those more regularly than everything else.