Unable to connect world issue

Hello, i'd like to report an issue that occur during joining friend's world. I have seen this issue everywhere, but yet it is not to be fixed yet. Apparently, when i join my friend's world in an online multiplayer, it says "Unable to connect world" without any specific reason. I have a licensed minecraft, so i dont think that is the issue. I have been searching for solutions in the internet, and there are a LOT of people that have the same problem as me and my friends since around April 2019. Please fix this issue in the next update, the team really need to fix this long-lasting issue in multiplayer since many players on mobile having the same problem.