Piglin transforms into zombified piglin underground in fully enclosed bunker without sunlight (Minecraft 1.21.7 Java)

When I keep a Piglin deep underground (about 50 blocks below surface) in a completely enclosed bunker with no sunlight exposure, it still transforms into a zombified piglin. This happens consistently even though the bunker is sealed with solid blocks and no light from the sun reaches it. No mods are installed, and this behavior is unexpected because Piglins should only transform when exposed to the Overworld daylight or being outside the Nether.