If your datapack overrides a minecraft/structures file, such as an igloo, this overriding takes effect even when the datapack is disabled.

Repro world attached. The world has a datapack that replaces the furnace that usually generates in an igloo with a sign. You are standing outside the igloo. To reproduce: run "datapack list" to verify that the pack is disabled. Then save and exit, delete all the files in the 'region' directory (so MC must regenerate terrain), and re-enter the world.

EXPECTED

you get the vanilla igloo generated in front of you, since the custom datapack is disabled

ACTUAL

you get the custom igloo (sans furnace) generated, despite the fact that the datapack is supposed to be disabled

ASIDE

igloo_bottom.nbt in Minecraft now contains two extra entities, post-flattening: a splash potion of weakness and a cactus, as item entities sitting near the brewing stand (containing another potion) and the potted cactus. this was surely introduced as a bug when the structures were updated for the flattening.