Strange networking issue possibly regarding Windows 10

[Somonkin]

Firstly, I apologize if this issue has already been addressed or covered somewhere else. I searched both this forum and the tutorials but couldn't find anything quite like the issue I'm having. After learning of the improvements to PPSSPP's networking (thank you very much to those who contributed!), I wanted to get back into playing some PSP games with my friends, Friend A and Friend B. The two games we've tried together so far are Kingdom Hearts: Birth by Sleep Final Mix and Phantasy Star Portable 2 Infinity.

The gist of the issue is that Friend A and I can connect just fine. We can see each other, play games with each other, etc. It's all perfect. Friend B, however, is suffering from an issue in which he can connect, but he never actually shows up in any kind of presence in-game. We first tried on myneighborsushicat, but then I hosted my own external ad hoc server so I could take a look at the logs.

Whenever Friend B joins, I can see his connection info (IP, Mac address and whatnot) just as I can with Friend A, but he never actually shows up in-game for us, nor do we show up for him. We've ensured that we're connecting to the same room and the log console prints this information as well (for example in Birth By Sleep, manual, room 1). We've tried having Friend A host and even Friend B host as well. Regardless of who hosts, the issue is the same: I can see Friend A and he can see me, but Friend B is somehow "isolated" despite connecting without any errors, and with the server clearly reading that he's connected.

I believe we've narrowed this down to an issue with his operating system. For context, I run Windows 7 (yes, I know it's time to update). Friend A and Friend B both run Windows 10 20H2. In case me running Windows 7 was somehow an issue, I had Friend A and Friend B attempt to connect to each other without my presence, and this did not work. Friend B can only successfully see us in-game if he boots into his Windows 7 partition; then it works fine. Therefore, it leads me to believe that this is some obscure setting on Windows 10 or something to that effect that is somehow creating this issue.

For some extra information, here's a list of what Friend B has tried so far to resolve the issue:

- PPSSPP Version 1.11.3

Updating to the latest Windows 10 20H2 version since he was initially outdated (failed)
Running PPSSPP as administrator (failed)
Various router settings (failed, but he can connect just fine on win7 so I imagine it's not a router setting anyways)
Allowing PPSSPP through firewall (failed)
Disabling firewall entirely (failed)
Having all of us use a VPN such as Hamachi (failed)
Various settings in PPSSPP's networking tab that we attempted to sync such as port offset, TCP No Delay, Forced first connect (failed)
Having Friend B connect to me/me to him without the presence of a third player (failed)
Friend B connecting to Friend A without my presence (failed)
Setting port offset above 1024 (failed)
Running PPSSPP on Windows 7 (worked!)

So once again, it seems that Friend B is only able to play with us on Windows 7, not Windows 10... despite the fact that Friend A is on Windows 10 and can play with me just fine. Note that Friend B has tried using the same PPSSPP install on both operating systems, so his settings are identical, but still, only Windows 7 works. Any insight, advice, or suggestions into this issue would be much appreciated.

Thank you!