After a Receive is complete ArchiCAD has to refresh the project, do some maintenance on it. Sometimes it modifies/updates information, which seem to be a Change, so it claims there are unsent changes.
Sometimes a send fixes it after S&R, but there are times when this doesn't help either.
It means ArchiCAD makes a mistake and if it still indicates unsent changes after a Send (or two sends), then you are in this situation. You can Close the project without loosing any data.
Of course we can call this a bug and it should be fixed. It is already on the list and this sensitivity continuously evolves, it is just not there yet.
Product Manager @GRAPHISOFT, Budapest