Have you tried re-creating the report? Sometimes that fixes issues and it might even give you better insight into what might be causing it if you notice it happens after doing a certain action or changing something in the report.
Other things to try would be to go back to the old column name and see if it changes anything. I would just mess around with the settings and see if anything happens. If the column only became wide after a change then you might be able to figure out what that is and not do it again.
One last thing is try a different browser to view the report and see if anything changes. I know it sounds a little crazy but according to QuickBase the browser decides the column width based on many things.