Problem using filter for custom ticket field on dashboard in explore

So the situation is pretty basic:

• I have a custom ticket dropdown field on our tickets called "primary category". The selections help the reviewer know what part of our product the ticket pertains to.
• This field is required upon solving a ticket.
• I made a copy of the standard Support dashboard and put a filter for this field on the first tab (Tickets).
• When I use this filter and filter on one of our most common categories, the standard "Tickets Created" tile surprisingly shows 0. 
• When I then select NULL in the filter, I am again surprised that we have 300+ tickets, a lot of which have been solved and have values in the primary category field. I'm using All history on the time filter since we've only been in for 2 months. 

Any idea why when looking at a ticket, a custom field has a value and correctly adds the appropriate tag, but then when you filter on this field for a value, that it wouldn't show? No clue how to go about troubleshooting this one, so any help is greatly appreciated. Thank you!