TrueHD no longer works?

If your TV is connected via HDMI-ARC then it never passed TrueHD (or dts-HD) audio. HDMI-ARC supports AC3/EAC3/DTS/PCM at 5.1 channels or less (EAC3 5.1 + Atmos is supported).

Even if it was using HDMI-eARC, TrueHD from TV based apps would still not pass, as TV manufacturers block it (most support TrueHD passthrough for HDMI attached devices, just not from TV based apps).

If you want TrueHD + Atmos audio and 4K HDR / Dolby Vision, you’ll need to acquire a Nvidia Shield Pro or Xbox One (I do not know if Xbox supports DV). They are the only streaming devices that passthrough TrueHD + Atmos audio. Plex for Windows or Plex HTPC on Windows will pass TrueHD + Atmos audio, but the video is tonemapped by the app to SDR.

Ideally, you would connect it directly to your audio equipment, taking the TV out of the equation for audio. For example: Shield ← HDMI → XMC-2 ← HDMI → Sony TV.

See [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k for additional information.

Regarding Plex transcoding TrueHD to stereo, that should definitely not happen. You should get 5.1 audio over HDMI-ARC. Hopefully Plex can fix whatever is happening with the Android TV app in short order (the issue BigWheel mentions).