Auto numbering series when adding books

Hello, I have a number of epubs which belong to a general collection rather than having a specific series order. They each have a "calibre:series" metadata entry, but do not have a "calibre:series_index" entry.

Until recently, when I added such books to Calibre, they would automatically be assigned the next available number (which is what I want, since it essentially sets the ordering by date of creation), but now they each enter as "seriesName[1]" instead. Did I manage to screw something up in the settings, or has the behavior changed? And if so, are there any settings I can adjust to get things back to the old behavior?

I'd really appreciate any help from the experts! Thanks!