I can definitely confirm this. If my quick testing yesterday is anything to go off, the root cause is the radar altimeter. If you bank the A-10 more than 45° in relation to the horizon, the radar altimeter does not work any more, without any regard for the angle of the actual ground.
Say you are flying through a valley and are banked more than 45° in relation to the horizon, but only like 10° in relation to the ground below your plane, the radar altimeter does not function. As soon as you bank back to less than 45°, the radar altimeter reads somewhere between -2000ft and -1000ft and quickly rises back up through 0 to the actual value. when rising through 0 to 500ft, the GCAS fires, as it should.
I think the radar altimeter is at fault.