I have a piece in F#Major with a transposed trumpet part in AbMajor. The trumpet has a solo with written chord changes. These chord changes also appear in the piano part. A lot of the trumpet chords have become double flats and I can’t seem to fix it. I tried transposing to perfect unison with “avoid double flats” checked. This had no effect on the trumpet chords. I’m using Dorico 3.5 on a Mac.
If all else fails, you should find that you can select the chord symbol on the trumpet staff, open the popover, and enter the desired enharmonic there: this won’t affect the chord symbol on the piano staff.