Adding to this as it seems related:
When a number of attributes are optional, and some are mandatory, it seems that when there are multiple listings on the site, out of which some have values for optional attributes and some don’t, and a user searches for listings, then only those listings are shown that have a value for this attribute.
While this may seem normal at first, I believe this poses a problem. Say one attribute is called “Numbers”, and there are values from 3 to 7 in some of the listings in a given category. Other listings in that category don’t have any value for it, since it is an optional attribute. As the user doesn’t have any way to filter for results with a value lower than 3 or higher than 7, they also cannot search for listings which don’t have a value for this attribute.
So when they click on the respective category in the left-hand side search bar and click filter, they are only shown the listings with a value between 3 and 7 in that category, even though they didn’t actively choose that and may in fact be interested in listings which don’t have any value for the attribute.
I believe this is counterintuitive, as the user isn’t aware that they are only seeing a portion of the listings in that category despite them only ever having filtered by category.