Nightmare Werepig never respawning 10+days after scrappy werepig's death

I did not do anything unusual nor had any mods, this is just a endless default world I created and played on my own. After killing the nightmare werepig (1st time), I killed the scrappy werepig(also 1st time, both before killing celestial champion). I then defeated the celestial champion and went back to the caves and killed nightmare werepig (2nd time), and then killed scrappy werepig(2nd time). Instantly upon defeating the scrappy werepig, the junkpile moved and I discovered another scrappy werepig buried inside. At that moment, there are two scrappy werepigs showing in my screen, one defeated and one still buried. After defeating the new scrappy werepig(3rd time), I never never saw nightmere werepig respawning again.