This is on Beta 20, but I'm pretty sure occurs in earlier betas as well.
When using "then" in a formatter, the inputted format will be incorrect when clicking save. Here is a quick example:
I create the following in the server data bindings window:
After clicking select on the "Data Formats" window (the above screenshot), this produces calendar_event_allday == 1.then('yes', 'no') in the server data bindings window code view, which is correct.
However, as soon as you click select to save this, it will automatically add a + to the formatter.
Thus, completely breaking it. The only way to fix this is by manually editing as server connect file in the JSON view to remove the + symbol.