Fully rebooting in Safe Mode, then restarting normally didn't directly solve the issue - the collection of photos were still corrupted as per the photo above.
Doing the flip and re-flip dance did clear the corruption on each once the thumbnails had rebuilt, but I'm not entirely convinced things will stay that way... that said, perhaps the fixes we've tried 'might' help prevent the problem occurring with future imports..?
Again, in the seven years and various OS changes since I started using Mac / Leopard / iPhoto / Photos etc, I've never encountered this issue before.
Considering the immediate history preceding and following the first occurrence of this issue, I can confirm that the camera is the same, the format and resolution are the same, the memory card is the same, the MBP card slot is the same... the ONLY thing which has changed recently is Sierra.
Complete coincidence perhaps, but it's a clearly identifiable change in an otherwise entirely consistent arrangement.
Will continue to use as normal and see how things pan out.