My HT also behaves the same way; In fact, TrueHD, DTS-MA, AAC are all supported only via HDMI1/2 and not via ARC. So, I have a firestick plugged in HDMI1 port and my Plex happily pass-thru the Audio formats.
Though my TV says it supports Dolby Atmos, if I play the same movie on TV and audio via ARC to HT, TrueHD --> DD and DTS-MA --> PCM Stereo, AAC --> PCM Stereo. So, it is better to have Firestick / any streaming box connected directly to your HT receiver.
From where exactly? I do not see this model anywhere online.
