Screen goes black dictating in landscape mode

[ftlum]

When I try to dictate (microphone button) in landscape mode (specifically with the volume buttons upward), the dictation cuts out prematurely and the screen goes black for a second. It seems to get triggered by subtle phone movement and interestingly does not happen when the volume buttons are downward. I'm not using a phone case and turning off Siri's raise to talk does not make a difference.

Is anyone getting this behavior (or not)?

thanks in advance,

- Frank