Replication not updating XML Schema collection
According to Publish Data and Database Objects in the section Considerations for Publishing - Limitations on Publishing Objects:


  XML schema collections can be replicated but changes are not
  replicated after the initial snapshot.


I agree, this appears to be a deficiency.

As a workaround, you could perform the XML schema update on the Publisher and then post the XML schema update to Subscribers using sp_addscriptexec.