I've noticed in fractals and WVW that Stoic Demeanor is applying it's effect to me as well when I get cc'ed, much like the renegade's Ashen Demeanor once did.
After further review, this bug is really inconsistent. In fractals, virtually all traps and panels that either cause immobilizes or knockdown will trigger the trait. I've uploaded the results from Swampland Fractals on level 5 which is the easiest way I've found to reproduce the bug. Your harder CC like daze, stun, and knockdown seems to work reliably, however, knockback doesn't seem to trigger it at all on me but it does work as expected when using the shield of absorption. I haven't tested its effect with wards nor have I had the chance to evaluate Condi/cc like taunt and fear effect. I've seen slow be put on my from a core guardian which doesn't have any slow skills so I know the trait is affecting WVW. PVP is likely also affecting but I don't play that mode so I cannot say.
The worst of this bug comes from mobs or large fights in WVW. Multiple targets can trigger the trait with you being shutdown and burnt alive in seconds which happened fighting the Claw of Jormag earlier. Ironic to die by fire to icebroods.