Extreme use case scenario: you can select the desired audio stream on the tv, hear the wrong one, continue on your smartphone to watch und hear the right one (Plex saves the audio selection, this is linked by your account and the video file).
I found other threads on the forum like this:
The problem seems to be with the last three generation of LG Webos. The guy in the linked thread uses a C8 from 2018 and I am using the B9 from 2019.
So, the Plex client does nothing wrong?