If you use burned in subtitles with Plex (hard coded subtitles, part of the picture) the tv has no problem depicting them when using HDR+ mode and standard or low dimming. Tested with Unforgiven on Plex with subs burned in. ( very dark movie )
When using subtitles that are an overlay added by an app (plex, some Netflix shows, some amazon prime shows) it always make them very bright. Tested with Unforgiven on Plex with subtitles not burned in selected.
This probably has to do with the tv thinking there are 2 sources that need to be lit up when using an overlay subtitle.
Samsung; Make the tv and subtitles act as one image instead of layered when viewing anything.