I further investigated the issue and want to share some insights.
It seems that the problem is related to streams sended via Firefox.
I testet the following browsers for sending video: Firefox Desktop,Edge Desktop,Firefox Mobile and Chrome Mobile
The following image shows the normal behaviour using Edge Desktop browser as sender:
When using Firefox Desktop as a sender it looks like this:
Especially interesting is the high bitrate even when sending LD quality.
Here the results using mobile browsers as sender:
Performing the same test with Firefox mobile reveals the following:
It seems that when using Firefox as sender the bitrate is not properly propagated, which could probably lead to wrong bandwidth estimations and even turned off video streams.
Moreover Firefox mobile uses the H264 encoder. When I remember right simulcast is not possible with H264. That is, it is not possible to degrade quality to save bandwith. The only option in this case is to turn the transmission off.
Would it be possible to
A: properly propagate the bitrate with Firefox?
B: prevent using H264 for Firefox Mobile?
I think both would produce a more consistent user experience.