Thanks for posting to the Community! ADA Compliance is an important thing that all businesses should consider and it is great to hear about Community members ensuring that their sites follow this compliance as well.
Upon further investigation, I have found similar cases where other users were experiencing the same issues you were describing. Appears that it has to do something with the non-legacy form; the overlay blocks content from receiving the keyboard focus, and the screen reader announces hidden content. I'm sorry I'm not myself a "real" developer so I'd encourage you to test this out to be certain — in regard to
a legacy popup form, you might want to have your developer insert/remove the form's containing "div" element via Javascript as opposed to hiding/showing the div via CSS. If the div is just hidden or set to "display: none" via CSS, the content would still be rendered "invisibly" in the page source, but inserting/removing the form code via Javascript would prevent that from occurring.
Another recommendation I would have is to indeed check if delay time is a variable that affects how forms work. In regards to Shopify theme being a variable, I would assume that most of the forms work the same in terms of code but I would have to get confirmation on that.
In the meantime, you may be interested in using our legacy signup form, as it does allow for a lot more control over the elements of the form. All of the Form Concepts put forward by the W3C Web Accessibility initiative can be accomplished using the legacy form.
You can learn more about this in the following help centre articles:
Hope this helped and thanks for being a part of the Community!
Alex