By any chance are you using bracketing and electronic shutter or mechanical plus electronic shutter? On my X-A3 with V2 firmware I get a black frame when bracketing if the scene is very bright. The strange thing is it can happen at 1/10,000 even though this is not maxing out the electronic shutter, I will get 2 frames with images and one frame completely black in the group of 3. So it might be something to do with the readout speed when bracketing.