Steps to reproduce (must be followed exactly):

Add an AudioTrack to the desktop (click and drag from the device panel)

Open the Audio Recording window (click the red button on the AudioTrack options in the timeline or open the context menu of an AudioTrack and select "Record Audio Region")

Wait for mixdown to render and click "Calibrate"

Open the dropdown for the Input Device by clicking the associated box (it should display your computer's default input device) and select any new device. This demonstrates that the dropdown is not disabled and you can select any other device any number of times by repeating this step

TO DISABLE, open the dropdown menu and close it by clicking anywhere outside of it. This works consistently, however I find that it can also be triggered sometimes without the dropdown being open to start (not as consistently)