Delay in beast capture

I've been running Einhar in my maps for challenge completion and I noticed both yellow and red beasts would sometimes not be captured despite being drained of HP. After a few seconds they will instantaneously regain all their HP. This back and forth process (I "kill" them, no capture, regain HP) repeats ~5 times until finally Einhar traps the beast. I'd say it happens maybe 30% of the time in a single map.