"Always Open With" Does Not Always Work

I've started running across an issue on my Mac in which PDF and JPG files, which I have set to open in Preview, sometimes don't open in preview unless I right click and choose "Open With."

It isn't all PDF and JPG files, but it is quite a few of them. Many of my PNG, PDF, and JPG files open just fine with preview by double clicking. But, there are many that don't.

If I right click and choose "Open With," the file opens just fine. But I have to do this every time.

If I right click and hold down Option to change it to "Always Open With" it'll open the file just fine, but once I close the file, it still doesn't open just by double-clicking.

I have two clues, but I'm still not sure what to do here:

There is a 1:1 correlation between the icons for the files showing the default Preview icon (vs a tiny version of the actual image) and the problem occurring. In all cases where the problem is occurring, the icon is the default. In all cases where the problem is not occurring, the icon is the tiny version of the actual image.

And, I've just discovered that I can't rename these files. They're not locked.

I'd thought it was perhaps a Spotlight thing, but not sure.

2019 MacBook Pro Running 11.2.1.