Another interesting little side-effect of choosing Process only updated subdirectories, and another illustration of why we warn users away from it....
Create a Minimal album with a few images at the top level, and a few in a folder.
Make the album.
Choose Process only updated subdirectories.
Choose HiDPI thumbnails.
Make the album. The thumbnails at the top level will now be 248x248 instead of 124x124. But the folder thumbnails will remain 124x124.
There is now no way to get the folder thumbnails to be regenerated. Even if you de-select Process only updated subdirectories, those 124x124 thumbnails will still be there. You're left with Force Remake, or Delete local album, or manually deleting the thumbs directory(s) in the output.
And just to make it more interesting, if you now add a new image to the folder, its thumbnail will be 248x248, but the others will still be 124x124.
I can't say how long this has been occurring, because I almost never work in this mode. If I choose Process only updated subdirectories, I'm always adding album objects, so the error isn't revealed.
Not optimal, but I think I'll leave that one.
I don't blame you. I was thinking about what would be required to take care of this kind of situation properly, and I began to feel woozy and had to lie down for a bit. Really, really messy.
The bottom line is that users should be choosing Process only updated subdirectoriesonly if they're not using a JSON-driven skin, and they're not making any changes other than adding a few more photos to the 2021 folder in the family album.