Plex doesn’t play dts or ac3?

Other than the audio being dts (Aladdin) instead of AC3 (Dumbo), the movies look similar.

It is not clear why Aladdin will not play. Your TV and receiver both support dts. dts audio should pass to your receiver via the optical audio just like the AC3 audio did with Dumbo.

Check TV Settings
What are the options / settings for Digital Audio Out on the TV? The online manual for your TV is not clear. It mentions PCM and implies there are other choices, but does not list them.

Ideally, there is a setting that passes both AC3 and dts automatically.

Try different Plex Client
Edit: This is to check if the movie file is OK or somehow corrupted. /Edit.
Can you play Aladdin with other Plex clients? Maybe Plex Media Player or Plex for Windows or Mac?

If so, what does the Plex dashboard show? Click on the gold rectangle with bars underneath (off to the right). This should expand the display to show if either the audio or video is transcoding.

Edit:
Try another video with dts audio
This site has many demo files, including with dts audio:https://thedigitaltheater.com/dts-trailers/
Try playing Animated Logo or maybe Orchestra Long. They’re short demos from dts and should easily direct play on your TV.
/Edit

FYI, we didn’t make any changes that would affect subtitles. Setting Display Information = On enables the on-screen display in Android TV - the information in the upper left corner when you pull up on-screen controls or pause the video.