UDP Source port preservation

[0xalex88]

I'm running a pod that uses discv5 for discovery on UDP port 9200. It communicates with a bootnode on the same port to kickstart peer discovery.
The problem: the packet payload advertises the node's IP and port 9200, but SNAT rewrites the source port on the way out. The bootnode sees a mismatch between the advertised port and the actual source port of the packet, and discards the peer.
Is there a way to prevent SNAT/masquerade from rewriting the source port on these UDP packets?