Part of the issue here is that you’re doing field updates inside of a field renderer which I believe is not recommended. One way that I often get around this, is setting values in a page load snippet. I would try setting your default value on the page load by adding an “inline” snippet which updates the row to a blank string.
(function(skuid){ var $ = skuid.$;<br> $(function(){<br> var model = skuid.$M('YourModelName');<br> var row = model.getFirstRow();<br> model.updateRow(row,'Your_Field__c','');<br> });<br>})(skuid);
