Had to install the Vessel, just to confirm what I am saying.
They are absolutely the same theme – same code, same set of settings.
It’s just different starting configuration.
And I do not have the problem you’re talking about on Vessel too.
The only thing which kinda makes sense is this – since your monitor is wide, the leftmost submenu can be relatively far on the left and moving mouse directly from the parent menu item to submenu would result in closing the submenu since yellow part of the way leaves the initially hovered-over menu.
But this was always the case…
But again – share a preview link to your broken theme copy.
Can probably approach it similarly to this – Dawn 13,0 megamenu dropdown is dissapearing after hovering - #2 by tim_1