I just tried placing a Subscribe Form module on our live dev site, visited it on an iPhone 6, and the success message shows up fine. Have you tried any other phones? Does it only happen on the iPhone 6?
Yes I also ran it on a test dev server and was ok - this is hosting out of my control - any thoughts on what may be causing it, so I can suggest something to their host provider.
This is now resolved - client was using it on a forwarded domain which was causing the issue - instead I redirected the domain to the source and is working as expected.