Background of main menu item disappears on hover over submenu

Hi, I'm looking for a solution to a strange menu behavior. When the user hovers over the main item, its background changes, but when the item has a submenu and we hover over it, the background of the main item disappears. Can this be fixed using css/js? Several users have pointed out that it doesn't look good.
https://gridbox.strefastron.pl/

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