[Ade1969]

1) no clue how you did this, but I've added some more points to write the NFO; I hope this helps

try it by yourself, this bug is still there. If you delete a movieset in tmm, the nfos of the movies before in there will NOT be cleaned. They still carry the information of the previously deleted movie set.
You have to delete the movie-set information in the nfos by open them in an editor. OR: NEU SCHREIBEN via context menu. Would be great, if the nfos would automatically be corrected & rewritten in this case.

Yes, I can confirm that removing a movieset (eg rmb on the set's name) in TMM 2.6.5 does not edit the nfo's of the movies contained within that set.
Of course, editing the movieset, highlighting and removing the movie(s), then removing the movieset, works ok... as expected.