Please help! Party Chat NAT Type Error, But Can't find a Fix Anywhere!

I'm having a super weird problem with party chat where when I join a specific friend's party chat it'll connect just fine, but I can't hear him, I then see his mic icon move every seconds or so as if he is talking but can't hear him? Then about a min into sitting in the party chat, his mic gets an 'x' icon over his mic and then a message pops up saying something about "it's possibly because of two different NAT types" or something along that idea. It can't be the NAT type because at first we thought it had to be the NAT type since usually I'm moderate NAT type and he's open, but after being annoyed for so long we both decided to give our internet provider a call to have them forward the proper ports to make us both stay open NAT type. So long story short, we hit a wall on what the problem could be and what to do to fix it.

TL;DR: I'm having a party chat error that says it's NAT type related, but we're on the same NAT type (open) and am looking for assistance. Thanks for any help!