PGS + TrueHD + Experimental PGS Rendering

[yorah]

Posting here following a post on Jellyfin forums: https://forum.jellyfin.org/t-transcoding-on-webos-with-pgs-truehd

Context

I have a mkv file (HEVC) with DV 8.1, two audio tracks (AC3 5.1 and TrueHD Atmos), and PGS subtitltes.
I am using the latest Jellyfin player build on a LG G4, hooked-up with eARC to a TrueHD capable AVR.
I activated the pass-through for TrueHD and DTs in the options.

Observations

1. Playing the file with either of the audio tracks results in direct stream => GOOD
2. By default, activating the PGS subtitles always results in transcoding => EXPECTED

I activated the "Experimental PGS subtitle rendering" option.

3. Playing the file with either of the audio track still resuls in direct stream => GOOD
4. Activating the PGS subtitles with the AC3 audio track results in direct stream => GOOD (seems like the "experimental PGS" option is working
5. Activating the PGS subtitles with the TrueHD Atmos audio tracks results in transcoding => UNEXPECTED

Am I missing something regarding behavior of 5. ?