It's not that Leviathan's HP resets, otherwise you should still have lost regardless because of the indefensible 2-hour attack that should have happened.

Rather as evidenced by your video clip, the entire fight reset. Leviathan you were fighting was replaced by a fresh Leviathan bringing you back to square 1 of the fight.

The latest patch/update broke a lot of things and caused a lot of mayhem. I am surprised to find that it extends as far as monster spawns.