Everytime someone tries to join my world over Xbox live it basically doesn't work. I can join them, but they not me with Xbox live and XBL is a must for cross-play. What I need to admit is though, that I changed decades ago once my netisolation with the following command (do not repeat, if you don't know what it does):

• CheckNetIsolation LoopbackExempt -a -n="Microsoft.MinecraftUWP_8wekyb3d8bbwe

I'm not sure if the problem is caused by this, I even uninstalled Minecraft and so on, no chance. On my friend with Win10 it works and at friends with a switch also flawless. Help is needed, maybe a undo command maybe.