ZDoom

That's what I'd imagine.

Just one question: if the victim is not exactly at the center of the explosion by 'A_Explode(1,..)', then the damage is always truncated to 0, right?

Hm, indeed, because the other wad in the ticket (exploderadius.wad) worked fine with that version. I'll bisect again and let you know.

No, that makes perfect sense. But that particular piece of code doesn't exist anymore, so it cannot be the cause right now.

Hm, according to the bisecting, the 'problem' was introduced since r3523. Sounds strange, doesn't it?

Also +CAUSEPAIN is now your solution.

Are you aware how much damage this attack causes? Precisely - zero! If that caused pain it may have abused a side effect of something, And guess what: There was a minor bug in P_RadiusAttack that let fractional damage slip through which got fixed between 2.5.0 and now. And if I remember correctly it got reported for the precise reason that it caused pain in situations where it shouldn't have.

See this zandronum ticket.

If you load exploderadius_02.wad, the monsters don't get any pain. It used to work in zdoom r2560 (or also in zdoom 2.5.0). Surely the damage code was changed a lot...