iOS Device - Getting "This form is not secure. Are you sure you want to submit it?" and stuck on Loading Response

We are having an issue in the test Moneris Hosted Pay Page. Using store1.

Using an iPad updated to iOS 11 and the Safari browser. When I fill out the payment info and submit, i get a popup message saying "This form is not secure. Are you sure you want to submit it?". I click the Submit button and I get stuck on the Loading Response screen and never return back to the app. I've left it for several minutes. We have the async response in place and so the order is eventually marked as paid through the async flow.

Any thoughts why this is failing? Is it a Moneris HPP issue? An iOS issue?