All i can do, is try to remember to make a video capture when it happens. I never figured why sometimes it doesn't show the bar and sometimes it does.
It doesn't happen very often, but from time to time.

2 capture where the bar is not there (don't know why, like at all)

0.9.8.73:

It's happens too often for me

One example:

This happens to me most of the time. Progress bar will disappear at some point during the capture. You will still hear the radio tuning audio, however. Its hard to tell if the progress bar stops because enemies have entered the zone or not.