Unfortunately in the current version you cannot override the instrument change label to remove this transposition, but it will be possible in the update coming in the autumn (the one that will have percussion, cues, and fingering support, not the one coming next week).
I have been following this thread and am wondering if there are any updates?
I would like to change Trumpet (B Flat) to just Trumpet. Has this issue been resolved now in Dorico 2? I haven’t been able to achieve this in Dorico 2 under Layout Options>Staves and Systems>Staff Label for parts.
You should check in Setup mode, in the Players list. Show the instrument card, Edit names. There, you can choose Show transposition : Never