As a Wix Designer, I have run into this Wix menu dropdown centering alignment issue over and over as I'm designing Wix sites. I would work on the Wix design, and build out new pages, then I'd run into problems with the dropdowns not aligning to the center of the tab that they're expanding from. See my example below:
When the menu was uncentered, I would try changing the Menu Text alignment, but I could never get it to center the dropdown menu elements. The Wix help articles I Googled say to use the aligned settings, which I have, but it wouldn't work.
Eureka! How Wix Helped:
I called the Wix Experts pro line, and a very helpful Wix support guy helped me get to the bottom of this. He said the menu element was corrupt, and to simply delete it and then re-add it through the lefthand Add + menu, so I tried inserting a new menu, and the the new menu's dropdown are better centered, see below:
This happens to my menus often as I'm building sites (has it happened to any of you?) I could never figure it out – but this immediately solved it, so from here on out, when I design sites, if the menu dropdowns get un-centered at any point, to simply delete the menu and re-add a new menu it to "reset" it's sizing.
The nice thing is that Wix keeps your most recently used menu Style at the top of the Add Menu window, so you don't have to re-style it if you choose the one you already styled.