Not completely similar, I have not seen changes in my playlists so far.
A few weeks ago I started to create a Collection named Best Artists.
Last week I saw it was empty and I could not add anything to the collection because of an error.
This and your issue with playlists make me question the stability of Plex and I will become more hesitated about using Plexamp until I know what happened. Unfortunately I have not seen any support at this.
I agree with what @Froberg said about playlists, it is too basic. No sign in an album what track is part of a playlist, no warning when adding a double track, no folders.
