When playing a Video the progress bar is not showing progress. I belief this is because the progress bar is thicker than the bubble and they both have the identical color. Thus the bubble is probably there and functional, it is just not visible.
In full screen mode everything is fine, as the progress bar line is much thinner.
I am sorry to hear that, would you mind opening an issue on the tracker: https://github.com/nextcloud/android/issues
That is the beast and shortest way to get in touch with the developers.