So I just got a Roku cause I read the 4K ones could finally output 24p, and got a Ultra 2016 (model 4640X, version 8.01 build 4042-29) specifically for the optical output. For now I'm using it at 1080p cause I don't yet have a 4K TV. I'm having trouble with 23.976p (23p) content though, cause the Roku always outputs 24 Hz, and there are periodic frame repeats every ~41 seconds, as one would expect (tested with the Pattern Tests video on Netflix). This is a problem because the vast majority of content is actually 23p, and not exactly 24p.
It also doesn't seem to output 60p, but only 59.94p, so in this case the situation is reversed and you can see one frame drop with the 60p Pattern Test every ~16.7 seconds, also as expected. But this case is not an issue because no real media content is 60p that I know of.
Again, these tests are done on 1080p. I would like to know if anyone has specifically tested 23.976 Hz output on 4K and if it works. 1080p seems to be a bit bugged, because for instance 1080p50 is not available as output, but 4K 50p does seem to work, so I'm wondering if 23p works on 4K and not in 1080p.