Hi at all.
I have two different site, with installed the same theme, same WP version same GP Plugin version.
I don’t understand how in one of this site https://www.realkombatsystem.it the menu html tag, the LI tags of principal menu, doesn’t contain the ID attribute. The other site contains it.
There is no particular plugin that, I think, alter the menu html.
Did you have any idea?
This would have to be a plugin or custom function on your site removing the ID. You’ll need to deactivate those plugins/functions one by one until you find the culprit.