does anyone know what causes this?

Could it be that you paused the game right after you drained his health?

I'm stumped on how that happened, but thanks at least for proving there's no way to skip the first boss fight at all.

Sadge this kinda proves the oob i found is useless

i always pause like that after i kill him, and the most pausing can do that we have found is slow down time in very specific time frames, and there is an explanation for that there is no explanation for this, i believe something in the memory fucked up somehow and caused that to happen, memory fuck ups like that have happened in the past to me causing wrong warps game crashes and stuff like that.