according to gst_mini_object_make_writable documentation:
   Checks if a mini-object is writable. If not, a writable copy is made and returned. This gives away the reference to the original mini object, and returns a reference to the new object.

if ReturnsModifiedPointer is not set, 
obj = obj.make_writable();
this lead to a reference count problem as the original object is unreferenced during the assignation.

this might also be the case for:
  - MiniObject.ref
  - Object.ref
  - DateTime.ref
  - Video.CodecFrame.ref
  - Video.CodecState.ref
  - Video.OverlayComposition.make_writable

I didn't enclosed a patch since it seems that this property can't be defined in a ".metadata" file