Volume slider in Garageband resets independently when using M Audio 49e keyboard.

I use an M Audio keyboard as a midi input to record in GarageBand. I have OS Monterey on an M1 iMac.

Recording into GB, I set the volume slider on the 49e and this is reflected in GB. Both during recording and on playback, the volume in GB repeatedly resets to a lower volume. Whatever settings I select either in GB or the manual volume slider, the output in the track is high, with notes pushing into the red zone.

I would like to disable the manual volume control on the 49e and use GB alone to control volume.

note - The m Audio 49e is quite old so it's possible that the volume slider itself is defective. If it is possible to circumvent this manual switch it would help diagnose whether this is the issue.

Any help or advice gratefully accepted. Thank you.