Mini bosses use PIGCOPSTRENGTH when respawning. While that could have been intentional so you don't have to waste too much ammo, I suppose it may have been a mistake when copying the code from another enemy, similar to how mini bosses also use AITROOPSEEKENEMY after the Shrinker effects passes. This was indeed fixed in the Nintendo 64.
That's a matter of balancing, not a bug. :\