Let me show you in screens. Look at borders in "More" menu and other tab-links.
Legacy vector (borders on the right and left of tab-links):
Current state of new-vector (no border on the left; border on the right):
Fixed border in new vector (and some other stuff):
So I assume you've intentionally removed left side border, but I think you simply forgot the right side. Hope that make sense.
CSS for removing the border (just for new vector):
@media screen { body:not(.skin-vector-legacy) #mw-head .vector-menu-dropdown h3 { background-image: none; }
PS: See also related reports for all other CSS: T286131, T286137