Heh. Nevermind, found the culprit, will post in a bit...
ps. So turns out, as the above behaviour surprised me a bit, that I had 2 playlists called 'WIP' in very different folder hierachies for different needs, as well as a static playlist folder called 'WIP' elsewhere, as well as a 'WIP' folder for smart playlist, and Musicbee did not like that at all... hence playlist edits being more than a bit erratic - funny though as some of them WIP named pieces have been around for a long time - I think I recently renamed the folder holding smart playlists to WIP, which might have been the straw that broke the camel's back...
Since Musicbee seems to be relying on unique names when it comes to playlists & folders, would be great if there was a warning when non-unique playlist/folder names are detected !
(in my extreme case that might be a bit tricky as I do rarely but occasionally do extreme edits to the organizational structure in file system directly as well, and let MB just read the resulting folder/m3u hierarchies... )
As well as throwing an error when a playlist can not be written (ideally with some indication as to why it failed).
Also, an interesting side anecdote... somehow, this whole case of non unique names for folders & playlists in MB also changed:
Preferences -> Library - playlists - exported playlists [ ... ]
path, which used to be:
Playlists_StaticExport\ (= *\MusicBee\Library\Playlists_StaticExport\)
and suddenly, while attempting to fix the multiple occurences of WIP names... I saw MB writing out all AutoPlaylists to new location, which was:
*Playlists\WIP\ ( = *\MusicBee\Library\Playlists\WIP\)
all of a sudden, without me changing it in the preferences...
('library playlists' is set to 'Playlists\', which remained unchanged)