Actually your debuffs can be hidden on the primary target bar as well if there are too many. There's a hard limit on the number of buff/debuff icons the game can display on a unit frame at any one time. Back in ARR this used to happen all the time when every class had at least 2 or 3... go in to a 24-man raid, good luck.

I'd like to filter certain debuffs out of the display entirely if they aren't mine. The problem is I don't want to filter out ALL the debuffs that aren't mine. I want to see if the boss has vuln up from trick attack for instance even if I'm not the NIN that applied it.