Nope, it's Stellar Panel and Starlight River that were causing the issue. Which is weird, because none of the code related to either of those items should be affecting it at all. I'll give Passive States another look, and I'll have to test it in the game itself, rather than in Battle Test mode, to see if it differs somehow.
Deferred damage stays the same when stats change; once the hit connects, the deferred damage is set. If you keep getting hit, that damage will rack up fast. I imagine a fast Block build with high Regen could make excellent use of the Chainshirt.