Render bug, timecode/footage shift on render

I've graded the project, source is ProRes 4444 .MOVs from Alexa Mini. Roundtrip with FCPX. Some shots are rendering with -1 frame shift, e.g. on the timeline the clip has IN: 19:44:09:09 OUT: 19:44:13:14, but on delivery it's rendered from 19:44:09:08 to 19:44:13:13. There's no time remapping on those clips, no transform, nothing. Just a piece of footage. Handles are off (0 frames). What the heck?