This has been a bug for quite a while. It happens with defending Awakened IMIWs, whether they have armour up, or not, under 15% health. I'm not sure if it happens in Practice mode, though.There have been numerous reports in the Bugs thread, and it never gets into the 'Known Issues' Trello.Maybe this time 😉
This is in 1.2 in the Thronebreaker EQ. He is gaining power below 15% without having any armor ups active on him. I did a practice fight against my own rank 3 IMIW and this bug did not persist in that instance.