I noticed this happening on my server a few weeks ago during a screenshare. The user webcam thumbnails at the right of the screen would alternate between showing their cams and being completely black. It seemed better when screenshare was off (so the participants’ cams were the main focus), but still happens. Could be a bandwidth problem with my server, in which case I’d have to know how to change what value where.
It could be due to bandwidth adaptation indeed. There is nothing to tweak the video for as many users as possible will be shown, sorted by last active speaker status.