XnView Software

Check in options: show folders as thumbnail.
The folders are automatically shown as thumbnails.

Uncheck in options: show folders as thumbnail.
The folders are stil shown as thumbnails.
Click on View/Rebuild thumbnails. Folders are correctly shown (as icons instead of thumbnails).
Click on another directory in the tree and then go back to the first one: Folders are again shown as thumbnails.

Yes, the thumbnail is saved in database, so rebuild is the only way

But rebuilding doesn't seem to help.

The folders are only shown as icons as long as you stay in that directory.
If you go to another directory and then go back to the first directory the folders are again shown as thumbnails and not as icons.