In another post asking about new things added to biomes I asked Valheim on Twitter directly whether an entirely new generated seed was needed, or if a copied seed would work and they said

"Most things will be available in an old seed without any problems, but there are other stuff that will only be available in unexplored areas, so it depends on how much you have explored."

Upon hearing that then this thought occurred with Sunken Crypts because that's the case with tar spawns with its biome, so it and Burial Mounds start to become explored upon entering them. With three people in three Crypts of five in the area we got around 360 iron each back in May iirc. In one Crypt at the entrance room with three branches one hallway was a dead end three way split blocked off upon mining through the mud scrap covering each rubble cave in, the middle entrance branched off two ways and then one route linearly for a good handful more, and the right entrance route went linearly down with close to a dozen rooms with mudpiles galore