Folder is not empty when trying to move indexed files into the database

I used to have a bunch of MD files indexed.
Some of them were into the group, others were indexed elsewhere in the database and taken inside the group by smart groups.

Now I selected all of them and chose Move into database, but there’s still the Index icon in the root group, and the log says “Folder is not empty”.

Is it normal?
How do I fix it?

Thank you!

CleanShot 2023-05-13 at 10.59.06@2x

When moving into database, sometimes some files or folders aren’t moved. I suspect they are temporarily blocked by an external application or even spotlight itself.

Normally, doing a new “move into database” from the context menu, remaining files are definitively moved into.

(Check the icon that shows if a file is indexed or not, to know all files have been moved).