Happening over here as well! In one instance, right clicking the blueprints in the content browser resolved the issue, but I couldn’t reproduce this solution with all of the glitched thumbnails. I discovered this solution after I had right-clicked a blueprint to sync it from source control.
I’ve been dealing with this in my project for a year now. It seems to come and go with about 10% of my blueprints - I’ve checked twice, there is nothing about the affected BP’s that explains it, etiher. It’s not game breaking, by any means, but it’s one of those quality of life fixes that I’d kick a baby to get.