Feature to sort by filename — where is it?

Does anyone have this feature on their Pocketbooks. I'm sure I've had this feature in previous firmware versions, but after the big software update to the Touch Lux 3 in March, it has apparently disappeared. I got this image from pocketbook support, who insist that the feature still exists. On my device the only items in this menu are sort by addition date, opening date, title and author.
I reset the firmware to factory defaults to no avail.