The much more likely culprit is your VPN server's port. If it's running on some no-name port (such as the default 51820), that's likely to get throttled.
I'd bet that switching your VPN server port to 443 would solve the problem, since HTTP/3 runs on 443/udp.
I'd bet that switching your VPN server port to 443 would solve the problem, since HTTP/3 runs on 443/udp.