See video. Brom does massive damage to beasts (they die), Solaris rezzes them, and jabber manages to fear away Brom... when that happens, their health bar goes back to max potential.
You can see my Solaris kills GT (and Grog) doing 166 dmg. When def Solaris rezzes them again, they seem to be back at full health (Ferno does 2400 dmg and doesn’t even dent GT)
I’ve seen this multiple times before, where the health bar changes after “unhealable damage”. In the past I noticed it when using Daeris/Zomm... it seems that when the hero who did the unhealable damage is removed (fear, turned into a minion), the “unhealable” restriction is lifted