Bastion Prismplate going on cooldown without activating

I've been testing the Bastion mercenary's Prismplate and found it triggers when taking physical damage. Using Echoes of Creation, I take damage and he instantly casts Prismplate if it's not on cooldown.

However, while in combat, Prismplate is never cast, but it seems to go on cooldown since I cannot get him to recast it using the same method until a certain time (up to 15 seconds) passes.

I tested using Unwavering Stance to make sure it wasn't caused by stun, but this didn't work.

Is this bugged or is there some other condition I'm missing?

Edit:
Some more findings:
-Equipping it with a Gruthkul's Pelt makes him unable to move
-Unequipping the shield makes him unable to move
-When immobilized (10 grasping vines), Prismplate was casted properly

Bumping and can confirm - using Leap Slam with Lifetap support while out of combat will trigger Prismplate casts with only a few seconds of downtime in between, but while in combat I'll go long stretches without the buff.

Was really excited to play with this Bastion merc, even put in a Runegraph of the Warp for max Prismplate uptime :( Any ideas GGG?