During playback of multiple streams from a Multisource stream, you can experience video freezing for the non-primary stream. This can occur when the aggregate bandwidth of the non-primary streams exceeds 4Mbps. Note that the primary stream is the stream where the sourceId is left blank.
The capping of aggregate bandwidth is by design. Each viewer instance has a bandwidth cap for non-primary streams of 4Mbps. In scenarios where you know your aggregate will exceed 4Mbps you should either publish with simulcast, or use a lower bitrate such that the total is not above the limit i.e. if you are going to display eight non-primary streams, use less than 500kbps for each.
Note that the Realtime Streaming API will not limit the number of tracks that a viewer is able to receive, but will limit the maximum bitrate per viewer across all media tracks. You should configure the Simulcast/SVC bitrate of all the sources carefully within your applications so they can receive the desired amount of video tracks in the viewer session.