Preview on MacBook won't open some JPG files, will open others. Happens with photos from iPhone 12.

First, I'll make it clear that this is an old machine (MacBook 2015) running an old system (High Sierra

10.13.4). I won't upgrade the system because some important peripherals, whose manufacturers are not supporting them any longer, will not run on a newer system. So I recognize that there's an obvious handicap here. Still, the problem seems unusual enough that I'm not ready to blame the system.

Problem: standard JPG files (like from my iPhone) will display just fine on all other devices I own. However on this machine, when I try to open them in Preview (v 10.0), some will open and some won't. On my iPhone 12, and on my iMac (2020 machine, running Catalina), ALL the JPGs display just fine. When I transfer them to the MacBook (via Dropbox, or via AirDrop), some JPGs will open, but most will not.

For those that won't open, I get the error message "The file 'IMG_nnnnnnn' could not be opened. It may be damaged or use a file format that Preview doesn't recognize." The photos are definitely in a file format that Preview recognizes, since some of them do open, and all are JPGs. And they are not damaged, because I can open them with no problem in Photoshop on the MacBook.

The only clue I can detect is that the JPGs that won't open are generally of larger file sizes than the ones that will open. 2 MB files will open; 6 MB files won't (seems to be a general rule).

The only other possible clue is that I did not have this problem until I got my iPhone 12. I have no problem with photos (of any size) shot on my iPhone 8 or my Nikon DSLR. Also, no problem with PNGs or MOVs of any size.

For those who may suspect this setting on my phone, I believe I have it set correctly: Settings

-> Camera -> Formats -> "Most Compatible" is checked.

Any thoughts or hints appreciated - thanks.