Fix: Merging mesh and non-empty BMesh creates "flag" attributes

We need to use the custom data copy function that skips mesh-only
attributes like the hide status attributes, the generic material index
attribute, etc. Otherwise the BMesh has those attributes which
conflict with their builtin counterparts.

source/blender/blenkernel/intern/customdata.cc

@@ -3751,7 +3751,7 @@ bool CustomData_bmesh_merge(const CustomData *source,
     destold.layers = static_cast<CustomDataLayer *>(MEM_dupallocN(destold.layers));
   }
 
-  if (CustomData_merge(source, dest, mask, alloctype, 0) == false) {
+  if (CustomData_merge_mesh_to_bmesh(source, dest, mask, alloctype, 0) == false) {
     if (destold.layers) {
       MEM_freeN(destold.layers);
     }