A week ago my listening activity stopped being updated for my friends and completely vanished from their side bars today, although I have been listening to music via Spotify almost every day. The settings remained unchanged and the same on each device, namely not having activated a private session and enabling "share my listening activity".
The last activity that was shown to two of my friends relates to listening activities on device 2, the next (unrecorded) listening activity took place on device 3. Since then Spotify was used to listen to music on all three devices, none of these activities was successfully broadcasted to friends, which seems to indicate a general account-related issue not specific to the client.
Besides updating the clients on all devices, logging out of and back into my Spotify Account and keeping the OS-es up to date, I have tried unfollowing and following affected friends again, yet even them doing the same did not help.
Many other "listening activity" community forum questions seem to relate to not seeing the activities of one's friends, which I seem to be able to do. Maybe this illustrates the same/a related problem from the perspective of one of the vanishing accounts.
Device 1: MacBook Pro, Mid 2015
Operating System 1: MacOS 10.15.7
Spotify Version 1: 184.108.40.2065
Device 2: iPhone 6s
Operating System 2: iOS 14.2
Spotify Version 2: 220.127.116.119
Device 3: Dell Latitude E7440
Operating System 3: Fedora 33
Spotify Version 3: 18.104.22.1686 (installed via snap)