Why does TCP port forwarding work but UDP does not?

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this is my first question ever here so sorry if it's not in the right category or not perfectly explained. I'll do my best to be descriptive and precise.

Basically, I've done some port-forwarding for games as well as for server-client testing purposes for development. In the past, it's always worked perfectly fine. However, recently, I realized some things stopped working for no apparent reason.

After quite a while of testing, I noticed it was an issue with UDP. Things that require only TCP ports to be forwarded work just fine, however things that need UDP ports forwarded do not. I confirmed this by testing exactly the same application (a simple server-client app made in Godot just to test this) with both TCP and UDP. It connects fine through TCP, but not UDP.

This has never happened to me in the past. Any ideas on what the issue might be? I've Googled for hours and the only things I could find is that A. UDP troubleshooting is apparently incredibly difficult (especially on Windows) and B. Some people vaguely mention the possibility of it being a problem with my ISP filtering UDP stuff.

I've already made sure to check it's not a firewall issue, either. Not sure what else I could try. Am I just missing something really obvious here? Thanks greatly in advance for any possible ideas or suggestions.

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udpportforwarding
Port-forwarding is a feature of NAT routers, the configuration of which is off-topic on Stack Overflow. You may consider posting your question on superuser.com, serverfault.com, or networkengineering.stackexchange.com, though you would definitely want to make sure you've done sufficient research before doing so, and that your question hasn't already been answered on any of those sites. – Peter Duniho

1 Answers

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You might not get UDP responses because server might be using that specific ports for TCP.

Different UDP scanners using different techniques may come to different results.

There is a possibility of ISP filtering UDP port only if there is hidden NAT -Meaning you will be assigned a public IP, but still would be reaching internet on a different IP (NAT'CEPTION), check for what is my ip to confirm this.

You can take a wireshark packet capture on an upstream device (Firewall or modem if possible)