iMovie turns some images green

Creating a new movie, and I'm finding that some, not all, images from Photos turn green when added to the iMovie timeline.

• All the photos come from an iPhone, and have been imported to Photos.
• Two photos from the same session will produce different results. Drag one in, it works, drag another which happened seconds later (chronologically), and it will appear green when played in the movie.
• I've updated all software to the latest release.
• I've restarted all apps and the mac several times.
• I've attempted to edit the image in Photos (cropping, rotating, changing filters, etc). Changes do get reflected in the photos library in iMovie, but the image(s) still turn green when being brought into the timeline.

So, I'm stumped. What can I do to make sure all images can be seen in my movie?