Fix clarinet tranposition key signature

There is no D# key signature (the enharmonic of Eb) for the clarinet to have as a transposed key signature. Since you must have a key signature with flats for the transposed key signature, the enharmonic Gb concert pitch key signature is used when you switch from transposed to concert pitch. If it did not do this, then nearly every note on the clarinet part would have an accidental on it when you entered notes then switched from concert to transposed pitch.

I hope this makes sense.

My guess is that since you are transcribing a Debussy song (I didn't look for the score), there is an A clarinet rather than a Bb clarinet in the score. Change to the A clarinet and this won't happen.

Use the concert pitch button

That is really annoying isn't it. It makes switching to and from concert pitch difficult as MuseScore doesn't automatically reinstate the 6 sharps after it correctly changes to the enharmonic key signature with concert pitch unselected.

To deal with your current problem: It is possible to have the score shown in concert pitch but the parts transposed. The concert pitch button works independently in the score and the parts. So you can enter everything in the score at concert pitch and as long a you never switch away from concert pitch you will keep its 6 sharps and you can unselect concert pitch in the transposing instrument parts and have the appropriate key signature there, enharmonically changed if necessary. If you do switch the score away from concert pitch after switching back you can re-apply the 6 sharps key signature to get back to where you were. But it would be nicer if MuseScore remembered the key signature you originally entered.

That is a workaround then

Yes, but it's not much fun having to reapply a key signature every time you switch back to concert pitch.

The problem seems to be known and recently introduced. #302566: Key signature not restored correctly when toggling concert pitch

The workaround is to NEVER change from concert pitch. If the key signature changes from sharps to flats and transposes the notes appropriately, then applying the other key sigature again will only make accidentals rather than showing the proper enharmonic notes.

This means you must create parts if you want to see the score in both transposed and concert pitch. You can create a part with all instruments (you must manually add each instrument) then switch between the concert pitch score and the parts rather than using the concert pitch button. I have no sense of how this arrangement will end up being done, but you need to be very careful if you find a need to use multiple voices in either the score or parts. Using voices can lead to corruptions. If this happens, delete the parts and regenerate them to fix the corruption.

You can change from concert pitch but as I said earlier, if the key signature is changed enharmonically when you change back to concert pitch you need to re-add the original key signature. MuseScore doesn't do this automatically.