Folder icons look like a blank page

A few days ago I used a third-party utility, Cocktail, to do a variety of maintenance operations like clearing caches on my late-2018 MBP running Catalina. (Last time I do that!) After this was completed, I noticed several folders on an encrypted disk image lost their typical folder icon. Now they look like generic blank pages. They function normally so this is just a problem with the appearance of these folders. As far as I can tell, the folders on my boot volume all look normal. Not sure why the folders on this disk image are affected.

In the image inserted into this post, you can see an example. The "Billing," "Correspondence," and "Progress Notes" folders are affected, while inexplicably other folders are not. I have several thousands of folders like this that are all affected. The parent folders containing these folders/files are also affected.

If I mount a backup copy of the disk image, made before this problem occurred, the icons are similarly affected. So it's not an issue with the files themselves, but some way the Finder is displaying them.

Is there some cache or some kind of file I can trash to fix this problem? Any ideas?

Thanks.