Picture ratio problem when playing from Plex

[BoldBelvoir]

I have a very strange problem, but to be honest, I'm not sure whether the problem lies with Plex or my TV (I'm leaning toward the latter and have also posted on Plex forums), but any pointers would be welcome.

Setup:
Plex server latest build, running on Win 11 PC. Plex client latest build on Sony XH90 (Android/Google TV UI). Hooked up via ARC HDMI to a Denon AVR.

Problem:
When playing certain media (either from my own libraries or streaming from Plex) the picture will appear as a small rectangle in the middle of the TV screen. Sometimes, when playing material from Plex servers, the aspect ratio even changes during the actual programme, e.g. the film will begin as a picture in the middle screen, go to an ad break, and come back full screen, but over-zoomed so that a lot of picture is missing off screen, then after another break, it may come back as normal (if you're lucky!).

This only happens when playing back on Plex. If I play the same material through, say, Roku or VLC, the picture displays normally. It also only happens on this TV. I have identical setups with two other TVs (a TCL C635K and another Sony W800 - both Android TV) and they both play back normally.

I can find no video settings within Plex (server or client) that may address this problem and when I try to adjust video settings on the TV itself it says I can only adjust picture settings with an HDMI source.

Any advice is truly welcome.

[HannahEd01]

Hey BoldBelvoir, as much as it goes, I can only see that the issue is within the Plex. But I know nothing about it or its settings. Contact their support, maybe?

On the TV, try the basics, unplug from the mains, remove external devices, update it, and do a factory reset. BTW, which model do you have? The full model I mean.