Anki Version 26.09.02 running on Linux (Linux Mint 22.3).
File Headers no longer appear to work. I use #separator, #html, #notetype, and #deck. They have been working for a couple of years now. But the HTML header no longer appears to be accepted. The only way to force the import in correctly, is to delete all the file headers and manually input the import parameters.
Even if I place the File Headers in alphabetical order (this was an earlier requirement of the import function), not all the File Headers are obeyed, especially the HTML one. Weirdly the HTML toggle shows as a light blue, not the deep blue it shows, when it has been manually selected.
I have not had to import these decks from CSV for a while, so I may not have noticed this change in behaviour, when it first occurred.