Can't join friend's session. (PS4)

Greetings.

My PS4 NAT type is 3, and my friend's is type 2. We couldn't join each other's session. We live in the same country (Malaysia).

However, i can respond to random people's SOS and they can respond to mine.

My friend once joined a session that was 3/16 full, and i tried to join but failed. But when he left that session, i was able to join it.

Also, i added friends from Canada and Japan when we met during a random SOS. The weird thing is I can join their sessions and they can join mine without any problems.

I recently Googled that NAT type 3 usually have a lot of problems when trying to play online, but how is it that i can't join a session with my local friend yet i can join my other friends who live in other countries?

Thank you in advance.