Did you notice the alert window that Audio inputs are dialed when using Airplay or Bluetooth? This window pops up when you turn Airply - Bluetooth on. So the behavior/restriction is normalHope that helps
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'
...in fact, I haven't noticed it, because I used to connect to the Bluetooth speakers via system settings. When connecting through system settings, no dialog shows up and returning to Garage Band gives You a "freeze" in the current amp-mode with all dials removed (just showing background "tolex").
Anyway: If disabling audio inputs is normal, there is no need to look for a fix. Just get an cable instead.
Does anybody know the reason for this? Possible HF-interferences with the JAM?