I rendered a short clip to XAVC-S with an audio fade to silence. With Audio Event / Properties / Loop checked, the audio waveform shows a 20 frame flat line after the loop point. What I hear sounds like what I expect.
@john_dennis Do you have Quick Fade Audio Edits turned on in Options?
Since the waveform doesn't update after edits, including looping edits which probably have quick fade audio edits applied, you would have to create a nested timeline, and look at the newly generated waveform (in lieu of rendering out the audio and bringing that back in) to see the actual waveform.
I brought the rendered file into a new instance of Vegas Pro. Since the side car file is created on adding the newly rendered file to Project Media, it is current.
The Quick Fade setting is at the default 10ms, which is significantly less than 20 frames at 29.97 fps.
The behavior appears to be the same as far back as Vegas Pro 14-270.
@john_dennis Well, I got out my Vegas Pro 15 and was able to produce the results you are getting in Vegas Pro 14. The only thing is the "bug" isn't noticeable until I am zoomed in on the timeline several steps.
But I can't produce the bug in Vegas Pro 21.