@jackie.chen This happens only when using an externalVideoSourceDelegate and
session.sessionPreset = .vga640x480
When setting the preset to:
session.sessionPreset = .hd1280x720
the starts just fine on start.
I think 
is somehow related to https://devforum.zoom.us/t/wrong-video-aspect-ratio-after-disabling-and-re-enabling-video.
Please look into it.