Boss HP bar randomly disappears

[Nessie]

So I was testing with “Big Red Kepa” outside Miner’s Village. Seems like if I use Cure / Heal then the HP bar will disappear. If I simply melee/auto-attack then the HP bar stays. I am a cleric using Mouse mode.

Noticed on my Swordsman alt that the problem does not happen in bosses, so not certain if it’s just something to do with ground attacks.

[xShinn]

when you’re fighting the boss, is your partymate a cleric? did he/she use heal on the boss?

It only happens to me whenever the boss was hit by heal. But other than that, it isn’t.