I went afk in my completely closed in base that does not have a single spot in it that is a viable spawn for any hostile mob and I came back to being dead with three pipers and 30 rats inside my base?!?!?

I've got this in a custom pack I'm making for 1.15.2. The main part of my base is underground at y=50, only entrance is a 2x2 shaft with ladders. As an example, while I'm digging out a new room, I might be suddenly attacked by rats and turn around to find a piper there. Light Overlay mod shows me everything is lit up. It does appear to be obeying distance rules; it never seems to spawn close to me. I've also had it happen above ground; I'm on a peninsula that I've walled off and torched completely. However, it does only happen at night on the surface, so it seems it's obeying sunlight levels, just not block light levels?

Also, FWIW, there are two other mentions of this on the Pied Piper page on the Rats wiki: https://rats-mod.fandom.com/wiki/Category:Mobs

Thank you, I'll be updating and re-enabling piper spawns tonight. :-)