I’m still having this problem, even after upgrading to 5.7.1.
It’s exactly as @room34 describes it: A select set to return “value”. On posts that haven’t yet been saved with the new value, I’m expecting no value at all, but getting a 1-length array with the first item as the value.
This is tough to work around, because we’re trying to introduce a feature without having to touch existing posts. I’ll post a bug report.