This isn't in Subset or Layout Settings, but if you have #LayID.#RevID on your titleblock it will achieve the same thing.
Then set your Publisher up with a Custom file name: <SourceID>.<CURRENTREVISIONID> <SourceItemName>
Maybe that was set up in your AC21 template?
Eli Chiasson
Dexel Architecture
http://www.dexel.ca
AC19-23 USA+MEP • i7-7700 3.6GHz | 32GB RAM • Windows 10 Professional