Sounds like a bug. What if you specify two classes? "columnClasses="panel-content,panel-content"? I know it's a workaround, but maybe it will apply the classes in pairs to all of the columns?
If I specify more than one CSS class(comma-separated) then system will assign CSS classes to the corresponding columns. So it works as designed.
The issue is that I want to write the column class once. The number of columns may change from time to time so it's potential danger here.