The change to this file 10 days ago removed a newline character at the end of each key added to it. This mean that when another key was added, it continued on the same line as the one before and thus wasn't being detected when an ssh connection came in with a key file.
With regards to https://github.com/mitchellh/vagrant/issues/7455 this is an (ugly) fix.
I'm sure someone knows a better command to concat a file and a string and append it to the `authorized_keys` file. But this does fix the problem.
Jonathan
Seth Vargo
Mitchell Hashimoto