Nada. I imported the media from the SD card in through Photos (don't ask why). Then I went and pulled ... mmmm ... I pulled the clips from the Photos folder ... which is imbedded in Photos.
Never again. That was dumb. I think I've just figured it out Tom. I'm going to pull all those clips out of the Photo 'library' into a normal (not embedded) folder, and ... nope, that's not it.
The clips were imported (dragged/copied) from that embedded folder (in Photos) INTO FCPX, where I've assigned everything to a Media folder in which is found the auto-created folder Final Cut Original Media ... so there is no connect with the original Photos material. Everything's been copied. Perhaps it has something to do with the H265 iPhone records in?