Hi Mike,
Thanks for sharing your survey. I have published the survey xlsx form you provided to AGO and have tested submitted surveys to the feature layer with iPhone X running iOS 12.1.2 and also with Windows 10 and Android 8.0. With all these platforms I can submit successfully based on your xlsx form and the current 3.1 release version of the app. I also tested with our 3.2 beta version and that is working fine too.
Therefore I can not replicate your error, can you try and uninstall and reinstall the app again, download the survey and then submit on the same iOS device.
Does this happen on any other devices, either iOS or Android or Windows?
Phil.