99% confident this is what happened.
I had Proteus’ debuff up on opponent’s left side (Rigard and a blue hero). Mana bar on both was grey. Caedmon charged up 2 turns later, I hit Rigard. The Proteus debuff became the red teardrop ‘bleed’ icon (I did not have a Barbarian on my team), and Rigard’s mana bar was blue again. Mana bar on the neighbor was still grey, and I had not even used my tile hit yet so it had definitely not run out.
Edit: Yes I do have Wu on my team. No, it was not active when I used Proteus. Again, both opponent’s heroes had the debuff and a grey mana bar. I do not recall if Wu buff was up on Caedmon when he struck, but he did hit.