Not really weird.
Consider this: QUIC is a whole new protocol based on UDP. Think, what happens when server needs to send the datagram but it has 7 interfaces? Which interface shall datagrams go through? It does not know that that packet is part of QUIC higher level protocol. It’s just a UDP packet. That’s why setting interface explicitly helps. You can see this for yourself with tcpdump.
Keeping port the same throughout the forward chain probably helps with QUIC path validation.
There is no reason to attempt to discuss or troubleshoot this any further until those configurations are made.