mkv->mp4 imported video duration and fps is wrong

I have an h.264 BD @ 23.976fps which has a "movie time" duration of 2:11:34:15 per vlc or any player.

I have an fcpx 23.976 project. it's important to note that the fps match.

I use makemkv to change from m2ts to mkv, then ffmpeg to change the container to mov or mp4 (either exhibits the same symptoms below). When I import this mov to fcpx or compressor, it shows the rate as 23.81fps. No other tools indicate anything other than the expected 23.976. huh. If I click on the clip in the library, the info browser shows a runtime matching

when i drop this clip onto the timeline, the duration is 2:11:26:16. yup, not the same duration as shown in the library. short by 7:23. and here's the punchline, fcpx does this horrible pulldown. it's a long pattern. every 17 frames grows to 24 frames. yet the resultant runtime is actually shorter, so not only are dup frames added, i believe some original frames are removed.

if you frame-by-frame advance in the timeline, you can see all the dup frames of course.

if i watch this mov (the original mov that was imported) in vlc or anything that can advance frame-by-frame, the dups are not there. this is definitely something fcpx is doing, i suppose based on the wrong 23.81fps it is picking up.

now, OTOH, if i use dvdfab in passthrough mode to convert from m2ts to mp4 (no mkv/ffpmeg middlemen), and import that mp4 into the project, the movie time duration on the library is shown as 2:11:26:16, and the duration when dropped onto the timeline is 2:11:26:16, and there are no frame insertions etc done by fcpx. IOW this all works exactly as expected.

only now as i write this post do i notice the 'rate conform' control. changing that changes the added frames, so for sure fcpx is misinterpreting the frame rate of the source clip and conforming it to 23.976. but regular players like vlc have no problem. any ideas what could be going wrong?

FYI I have discovered that either ffmpeg or the mkv format itself is the problem here. I've tried may combinations and ffmpeg never gets the output fps correct when the input is an mkv file, no matter how that mkv was generated.