I reverted my Kobo to factory defaults which formatted the unit and tried to reproduce the problem.
The problem definitely reappeared. I think it has maybe something to do with the metadata plugboards.
I used the steps described here (
https://www.mobileread.com/forums/sho...d.php?t=193184) to include series order on the titles of my books (which is great with shelves).
I used {series_index:0>2s|| - }{title} -> title.
So maybe Calibre can't match all the titles now (though it definitely works with most of the books in the series, always the same ones are missing).
Is it the intended functionality ?