Piano score where piano accompanies a solo instrument, cross-part oddity

I don't think of this as a bug report, because I am not reliably able to duplicate the actions and results. Perhaps it is a request to understand how Musescore behaves under the circumstances, so I can avoid the problem, or can fix it if it occurs. A 'user error' issue perhaps.
In a work for cello and piano, I have created the 'master' .mscz file. From that I have created two parts, The first is the 'piano score' showing the piano part and the solo cello, on a separate stave above the piano. The second is the 'Solo Cello' part. In the piano score I want the cello stave to be small notes, as that reduces white space and allows more piano staves to be on a page, but the pianist still sees the solo part.
As I see it, my options are to create the 'master' file with normal sizing for all parts, then create the parts, then modify the 'piano score' cello stave by selecting the cello stave and choosing all the notes to be small, using the Inspector.'
I have done this, with a result that the notes in the separate Solo Cello part are also now showing as small. Changing the notes in the Solo Cello back to 'normal', seems to then make the same change in the Piano Score.
I have tried making the notes in the cello stave in the 'master' file small, and then try to make them 'normal' in the Solo Cello part, but....the notes in the Solo Cello part are not shown in the Inspector as being 'small', so I can't enlarge them to be normal. There have also been variations, where some phrases are shown by the Inspector as being 'small' (so they can be enlarged), and others, even one next to each other, are not shown as 'small' so they cannot be enlarged.
I think this may have been a corruption of some kind, rather than a bug, but I welcome advice. What is a correct sequence of part modification in order to achieve a 'small note' stave in one part, but not the others? If this occurs perhaps due to a corruption of some kind, what might be an approach to fix the corruption, short of re-entering all the notes?