My son and I play together with him using his Nintendo Switch and myself using the Windows 10 PC bedrock app. We have both Microsoft accounts and Nintendo switch online memberships and he successfully logs in into his Hotmail account from the switch. If he creates a new world on the switch, he can invite me without issues. So it seems that there is no issue with the general capability of playing cross-platform.

However, on his Switch there is a creative world which we built together over several months. 

Since two/three weeks (I am not exactly sure), when he comes in that world, I cannot see him as a "friend" and he cannot see me - so he cannot invite me. For a while, he didn't seem to be able to invite me, but I was able to see him as a "friend" and thus join him. Now even that doesn't work anymore.

The issue is specifically for that world. New worlds allow invites and me joining.

I have tried changing a bit of settings in that World, including signing off the account on the switch and signing in again, and clearing up data, but nothing seem to make a difference.

It seems as some data is corrupted in that world. Is that possible? 

Can u advise on how to restore his capability of inviting me to that world?

Of course I can provide any specific diagnostic data you may need.