Can confirm, it still doesn’t work on Firestick even without the height parameter.
Not about to get an Nvidia Shield just for this, but it looks like there’s still a roundabout way to do this on Firestick. Need to use TinyCam or some other app that can play RTSP streams and also allows for starting up in Picture in Picture mode through ADB Intent commands.
VLC doesn’t seem to have an command to open in PiP mode. I was able to launch VLC through ADB with the camera stream, but it opens fullscreen. TinyCam might work, but it seems a little involved for me and I can’t commit more time to it.
Might give it a try in the future though.