rMWd7905627fdc3: resourceloader: Async all the way dropped the mw.loader.state( {"user": "loading"} ) call for the user module. Consequently, if a call is made to mw.loader.load or mw.loader.using for the user module before the user module has actually arrived, ResourceLoader will request it, and it will do so using the (wrong) version data from the startup module.
This primarily affected wikis that have modules in the default load queue that depend on the user module (or that call mw.loader.using( "user" ) from e.g. Common.js).