In case you didn’t find a solution yet, it could be happening due to having enabled “Allow Direct Play” in your Settings > Video.
For some reason, it forces the Direct Stream and this is causing playing your first audio track which would be the Portuguese one and you cannot change it to any other audio track.
This is happening to me on a QE65S95BATXXC from 2022 as well but, not happening on a QLED Q60 from 2020 so I can’t help but think this is caused either by Tizen 6.5 (2022) not allowing the picking of the audio track via Direct Stream to the Plex Engineers or it changed the way this is handled at the code level and we need a software fix for the Plex for Samsung app for 2022 models.
I have been trying as well pretty much every permutation on the settings for my TV just in case any Intelligent Mode or Dolby Atmos enabling or anything is messing around with this without any luck.
Additionally, I have seen other posts in this forum with a similar issue on Roku a few years back and, regarding the Plex for Samsung release notes, doesn’t look like there is any activity at all on software patches for 2022 models, something that, considering we are in 2022 September, looks pretty weird to me.
Hopefully, we will get some insight from the Plex Engineers 
