I used edit channels to clean up a mask for this image. Thought perhaps this might be the problem but tried what I am describing on another image with the same outcome.
It seems that latest beta will move to Tone Mapping but then doesn’t work. Also tried without success to change from rgb(16) to rgb(8) and rgb HDR (32) that was very dawgy. Video is long but fast forward past the saving issue to see that although it saved transparent, was able to use history to recoup the image. Included whole screen capture to show the length of time the process took.
RPReplay_Final1600235021.mov