Originally Posted by chaley
The default in preferences is
OK, this will confirm that setting regex to defaults mostly fixes things. I say "mostly" due to the oddity of text file metadata. I vaguely recall that calibre attempts to divine the metadata from inside the text file rather than always getting it from the filename. If the "metadata only from filename" option is off, it is getting the metadata of a txt file incorrectly on the reread. Apparently calibre never writes the correct metadata internally into a txt file, so with that option off, on the reread of metadata, it's always wrong. Other formats have correct metadata written internally, or if not, the metadata is picked up from the filename on the reread, so I saw errors only with txt files after the default regexes were set.
Turning on the "metadata only from filename" option fixed the txt file problem.
I'll check the delete issues now that this bug is at least identified.