MuseScore Now Always Prefers Sharps; Ignores Staff Properties Setting
Create a score with multiple parts, with instruments of different transpositions.
Set up the key signatures such that one part is in a sharp key, while another part is in a flat key.
Make sure "Prefer Flats" is selected in the Staff Properties for the parts that prefer flats.
Write some notes in the sharp part.
Copy and paste them to the flat parts.
Note: This may possibly also have used to work with chord symbols, unsure.