The problem was uploading it the build to Steam from Windows - it screws with the packaged app, something to do with file permissions.
I fixed it by uploading from a Mac using the osx version of Steamcmd that comes with the sdk. This might be worth adding to the UE4 Steam docs perhaps.
Apparently you can still use Windows to upload if you do this ContentPrep stuff, but it looked a hornet’s nest to me so uploading from a Mac is OK for now.