I have checked all the release notes between 15.1.8 and current release and saw no mention of a fix for this. I can recreate it in the demo app.
If you press the backspace key when editing a function in the function arguments dialog (fx button) it sets focus to the formula bar at the end of the formula and if you keep backspacing it will erase the entire formula, or if you start typing it gets it wrong.
See the screencast video for demo, you can see the formula bar getting focus after the first backspace.
http://screencast.com/t/fpZwqEGpp
You can also recreate it when adding new function using the dialog, if you type something into a field when adding a function (eg add ABS and type in a number), set focus to the formula bar, then click in the number field in the dialog and backspace, same issue.
Edit: It seems the actual issue is if the changes made via the dialog make the formula less than the length it was at the start it goes wonky.
Hi,
We have reproduced this problem and passed this ticket to our developers for further research.
Please bear with us. We will notify you as soon as we make any progress.