Default parts not deleted when instruments are deleted or replaced?

I have a score with 8 instruments - but it now appears to have 12 parts. They all appear to be default parts that have been created automatically as none of them has an option to delete.
I'd successfully transcribed a piece for 10 viols and had tweaked the styles, barline types, etc, as I wanted them. The next piece I wanted to transcribe had 8 viols and very similar tweaks. I though I might save "tweaking time" by taking a copy of the 10 part piece (I used save as) deleting the notes to get an empty score, then deleting a couple of instruments entirely and replacing the instrument on a couple of the remaining staves.
It rather looks like deleting the instruments didn't delete the default parts and that replacing the instruments created new parts without deleting the old ones.
Anyone else having a similar problem? I fear I've found a bug. Possibly related to #341808 but I've not found anything quite the same in the Issue Tracker.
Musescore version details are as follows...
OS: Windows 10 Version 2009, Arch.: x86_64, MuseScore version (64-bit): 4.0.1-230121751, revision: 9b70a8c