I've the same problem: HDMI-Grabber with Loop Out: MJPG only.
(on a Pi Zero - everything directly connected -- FireTV-->HDMI_Loop_Grapper-->TV )
already searched the internet/forums and read a post from 2017 that 'gstreamer' can help. whatever it takes, can this be enabled ??
(I am on Hyperbian 2.0.0 alpha 9)