The mask is only used if it's not zero. Adding the normal mask made it not zero, but it didn't include anything else, so all custom data layers except normals were removed. The fix is to only add normals to the mask when it should be used.
The mask is only used if it's not zero. Adding the normal mask made it not zero, but it didn't include anything else, so all custom data layers except normals were removed. The fix is to only add normals to the mask when it should be used.