Unable to join my friend's world and vice versa.

Dear Mojang,

Recently I have been noticing an issue which I have never encountered on the previous versions of the game, neither "Village and Pillage" nor "Buzzy Bees". I have tried many possible solutions to solve this but none of them had any results. I may have NAT type strict but my friend has Open Nat type and used to be able to join me and vice versa. We tried re-adding each other, restarting routers, the game and many other things but it does not work. I believe this is a bug and it has caused many problems with multiplayer experience for me. I hope a solution can be found and be fixed if this is a bug.