https://jsfiddle.net/cu1hr3jp/
I'm in desperate need of fresh eyes, I've spent about 10 hours trying to find the issue and cannot. In the above fiddle the columns do not align and it appears to be due to ScrollX and ScrollY -- if I comment them out everything aligns. In the past I know there were issues using ScrollX & ScrollY with fixedColumns, but I thought that had been fixed. Plus, I have at least a dozen other reports that use the combination of ScrollX, ScrollY, and fixedColumns and they work perfectly.
To paraphrase Princess Leigh: Help me DT Forum...you're my only hope...