I have had some issues with NPC filters atleast. You try to sort them using planets but it includes other locations aswell as that you can not "browse" properly between the pages if you have more then 100 NPCs to check.
Same seems to appear if you sort them on manager/supervisor
I imagine what Tyr described was the problem I'm used to, presumably when I was filtering for a smaller NPC inventory there were less locations to confuse the filters then with the larger. Or I was just talking crap, one of the two.
Ah, the 100 thing. Unfortunately there's no way to fix that without a rewrite as it's an issue with the core of the filter system. The best way around it is to click through each page (the filters are attached to them) and you'll come across them eventually. Not exactly practical for massive inventories, but better than nowt.