Boss fight - boss max health is displayed as 49,671

This occurred on a Nightmare Deep Creature fight that started "naturally" (not with a boss egg) The boss health on the scoreboard is listed as 49,671, and the boss bar also shows it as if the max health is much higher than the intended 12000. This seems to be visual, in that the boss DID spawn with the intended 12k health. I'm not sure if this occurred at the beginning of the fight or during (only noticed when another player pointed it out). I'm also not sure how to replicate this (only guess is if it's somehow specific to a natural fight)