Prior to 0.45 being released to stable I had my Z key bound to the 'Go Prone' action.
Upon logging in once 0.45 had been released I noticed that I could no longer go prone when pressing the Z key.
Clearing and readding the Z key binding to either 'Go Prone' or 'Prone' made no difference.
Assigning 'Go Prone' or 'Prone' to a completely different assignment (2x C) made no difference.
Resetting all key bindings back to the Dayz preset DID work and assigned 'Prone' to the Z key which allowed me to use the prone stance again.
Changing the Z key binding from 'Prone' back to 'Go Prone' after using presets results in loosing prone again.