I'm pretty sure Google's WebRTC doesn't respond to icmp needs frag (but it does have a better default size), and I'm guessing webrtc-rs doesn't either. Seeing the icmp in tcpdump might have raised some alarms.
WebRTC also uses small packets for ICE pings, so if you have a path mtu problem, it won't affect connection selection, so that's also fun.
WebRTC also uses small packets for ICE pings, so if you have a path mtu problem, it won't affect connection selection, so that's also fun.