I found FILMMAKER MODE (why is it capitalized in settings?) dark and muddy on a Samsung Frame. The "Samsung TV picture settings" section in the linked which.co.uk article [1] seem like decent advice.
It's dark because OLED are not bright at all. Anything brighter than the filmmaker mode is modifying the source image to achieve it, or alternatively driving the pixels in a way that loses color accuracy.
If you care about film and getting the closest result to what the actual thing is supposed to look like you're going to need to couple correct settings with a light controlled room for best results. Or don't use OLED, because, it simply can't achieve the brightness of cinema projection, not even close.
Personally I like the results of a 120hz OLED so much better than other options that I strongly favor a light controlled space for movie watching. For lower grade junk it's usually easy enough to swap to another viewing preset that is brighter.
Typically, it means that if you put the TV in a dark room, it is calibrated to the same specifications that the monitors used in post-production used. Therefore, it is what the directors "intended" the video to look like since they were looking at the monitors (in a dark room).
However, if your room has even a little light in it, the settings would make the TV too dark.
It will also disable any effects the TV has that aren't "map video to screen 1:1" such as motion interpolation, upscaling algorithms, etc
I can't find any explanation of how it actually works. Does a each movie get different settings set up by the director?! Doubt it.