Cannot adjust volume on imported midi tracks (Garageband iOS)

I imported a midi file with multiple instruments into GB on iOS. This works, but I cannot adjust the volume on these tracks. When I import the same file on my Mac, the volume can be adjusted without problems.

I tried editing and saving the midi file with MuseScore Studio, then importing it again, but the problem stays.

If I add a new track in the same project, the volume is also adjustable.

Is there a way to change the volume of imported midi tracks? Or is this somehow not supported on iOS.

Thanks in advance.