It could be a windows quirk, but my suspicions would lie with that single vertical grey line you have on the left side of the header. Since that object resized as well as the header, I would uncheck the bottom anchor for the positioning of that line and then see if that corrects your issue.
You should also make sure that no lines or other objects are overlapping part boundaries.
Correct. I did not see that it was 1 pixel overlapping and the bottom anchor was set for that grey line. Thank you, Mike )))
Good to hear it wasn't a quirk (those are impossible to debug sometimes), cheers!