What's my problem?

[ClearlyNotNeo]

that's quite a post and i can clearly see there is "a" problem but i fail to see what it is from all that information

start with what client is failing, they are not all the same and each can have quirks

All. I used Swiftfin or Jellyfin for iOS, JellyFin Web Windows, Jellyfin for Mac. All seem to fail.

next what it ISN'T doing is playing a 1080p movie encoded in h264; but what IS it doing instead? buffering, stopping, skipping, flashing, bad colors etc?

Buffering.. just the spinner

is the client trying to use direct playback or not?

Tried both, same result

what audio codecs are in use; f.e. my device will freeze due to DTSHD-MA but has nothing to do with video

Hmmm it's the default one never checked, never thought to check thanks for the idea.

are there any subtitles in the video; sometimes these can cause issues

Yes they are, didn't know that was a possibility, will try turning off

mediainfo output for the video could answer a lot of that
providing a server log can further provide answers
and if there is an ffmpeg log it will be most informative about codecs etc; if not then a mediainfo output would be useful

Will gather all the information regarding this

be sure all personal information is redacted, api keys, ips, domains etc. but please use the word "REDACTED" as it will be clear that it is there and not lead to a misconfiguration guess

Gotcha! Thank you for patient and time. Sorry for the rant post. Will gather everything related to that.