No matter how hard I try, I cannot get recent (as in last 5 years or more) versions of calibre to fall into this rabbit hole.
The other day I noticed I had failed to delete the ':' from a Title when I stripped the strapline into a custom column. When I tried to remove it using F2 in the booklist a message flashed (i.e. came and went), and the ':' was still there. I had left the book folder open after converting the PDF to DOCX with Word.
Previously (i.e. 5 or more years ago) I would have ended up with an extra subfolder, and a mismatch between the database and the library sub-folders.
BR