Is there any attribute that all of these images that show up as pixellated have in common
Yes! I've done another Full Sync and it seems to me that any raw+jpeg images appear pixelated when the files are off-line. Interestingly, I expected some old Olympus ORF files to show the issue as I thought the preview in those files was small but no, they are fine. But recent raw Fujifilm RAF + JPEG are not. Fujifilm RAF files without a JPEG are fine, the preview shows up as expected, but as soon as I select an image with a JPEG it renders pixelated.
I have PM set to show the raw and JPEG file combined in the same thumbnail on the contact sheet.
I checked the size of the JPEG as I expect you will say PM+ is using that file - not the raw - to create its preview. Most are around 6000 x 4000 px, so big enough to create a decent 1600 px preview.
So it seems to me an issue when having raw+jpeg files. I don't seem to have any older CR2 or NEF files with JPEGs to prove if this is simply a Fujifilm RAF issue, but why would it be? Any raw file be it CR2, NEF, ORF or RAF is previewed perfectly as expected when the files are off-line.
Thanks!