Recently, I can no longer see my friends listed in the Friends tab. It shows no friends online, even though I know they are. They use an Xbox One, Nintendo Switch, or a Windows 10 PC. And I use a Windows 10 PC.

As a result, I have no ability to join their worlds. I used to be able to just fine.

Note, however, that we can all play together on any of the Servers (e.g., The Hive).

Example:  One friend is on a Windows 10 PC. I appear on his Friends tab, but he does not appear on my Friends tab. Also, he can send me invitations, but I never get them.