FWIW - this is a very very long standing bug in Qt that is related to allowing use of qss files.
Here is the official Qt Bug Report that describes the issue:
https://bugreports.qt.io/browse/QTBUG-61092
So there really isn't anything I can do to fix this. Luckily this is only cosmetic.
Sorry. Every time we upgrade Qt we fix old bugs but get just as many if not more new bugs and this is one of those.
Note in MainWindow.cpp we use setStyleSheet on the Tabs to make them cleaner looking which is why this bug still exists even in Qt 5.12.1
KevinH