Looking at a Treated Wood slab from Immersive Engineering (edit: seems to be any IE slab actually) caused a crash (REI is also installed) -
Error: java.lang.IllegalArgumentException: Cannot get property EnumProperty{name=type, clazz=class net.minecraft.world.level.block.state.properties.SlabType, values=[top, bottom, double]} as it does not exist in Block{additionalplacements:immersiveengineering.slab_treated_wood_horizontal}
crash-2024-04-16_21.22.12-client.txt

This should be fixed now, though I'm still unsure why it happened in the first place, and it could still occur with stairs. It's probably an issue with REI improperly assuming a block's returned itemstack's item is the same block as the original block.