Typo in the first sentence – apologies. It DOES work with the plain 2.0.4 theme loaded, but not with the Child theme added.
After extensive iterative test changes, it is not in fact any customization in our Child theme, but something in the Ubermenu configuration (Primary) that is causing the conflict.
For example, using the original Pro 2.0.4 theme with my already configured MainMenu menu (and associated Ubermenu config), the form breaks.
I then created a single item simple menu (no Uber configs) and configured this as the top menu, and the form starts working.
I then re-enabled the child theme, keeping the single item menu as Primary, and the form works. This means the issue is in the Ubermenu config.
Doing some additional testing, I isolated one component potentially causing this: We have one Ubermenu menu item using the Ubermenu custom content type, which includes the following (used to enable our site search provider:
<form method="get" action="/search">
<input type="text" name="addsearch" placeholder="Search" autofocus />
Removing this from the Ubermenu custom content item allows the form to populate using the child theme and full config.
However, there is nothing unusual about this HTML inclusion in the Ubermenu Custom Content – I have used this with several other themes without issue, and is fully supported by Ubermenu.
I cannot operate or launch without enabling this search function, so I need Theme.co to determine why Pro is causing this conflict.