Debuffs always affect the true value, never the capped value. The game then limits you to the cap if you're still over, but going over caps can help to protect you from a few debuffs.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
I played from release till just after the kheldians were added, forget what issue that was. If I recall correctly, back then, it was advisable for a tank to go over the cap on their defenses, if possible, so that if they got debuffed they were still at cap value. Are they still calculating debuffs from the true value or do the debuffs start calculating from the cap value irregardless of what the true value is?