tvOS 11.3 HDR content on non-hdr tv is broken

Hi,

I have 2 Apple TV 4K boxes. One is connected to a 4K HDR TV, the other one is connected to a regular 4K tv which doesn't support HDR.

I have a small but growing library of 4K HDR movies (blu-ray rips) served on a local plex server. Before tvOS 11.3, both Apple TVs could play the HDR movies on the Plex app just fine. You would naturally lose the HDR effect on the non-hdr TV, but the movie would still look good.

With tvOS 11.3, the HDR content played through plex on the non-HDR tv is completely broken. There is lots of noise, and the colors are totally blown out. Anything remotely bright is pure white, and anything dark is pure black.

Interestingly, I have also purchased some iTunes movies which are available in 4K HDR. I tested with "The Revenant", and it plays fine on both TVs with tvOS 11.3.

As far as I understand it, Plex for Apple TV uses the stock media player shipped with tvOS. Are there some system-wide setting that I need to apply to get HDR content working with non-HDR tv's again? If not, is there a way to downgrade to tvOS 11.2?

Thanks.