Recent changes to Phoenix testing?

This seems to be a bit random. I’m getting issues with Elixir testing. The culprit line is this:

:ok = Ecto.Adapters.SQL.Sandbox.checkout(Draft.Repo)

It used to work and has suddenly stopped working. Not even sure what I did to get it to randomly stop working. But the error is:

** (MatchError) no match of right hand side value: {:already, :owner}

What checkout/1 recently changed to return something different?