I installed JA Tabs and found they did not allow the Megamenu to properly format. I tried different template styles but it was the use of JA Tabs.
I’m using latest Argo template and Joomla 3.8.3.
Sorry I had to discontinue the use of tabs or I could direct you to the Megamenu wrong display. Maybe it could be something with the width of the tabs as Megmenu looked like when the magnification of the browser was too big.