Hi Derek,
The reason for this issue is that on iOS 13 the MIME type of "application/*" won't work, which will happen when no file types specified in the xlsx form. For a workaround, you can specify one or more types in the body::accept column in xlsx form, i.e.
application/pdf,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document,application/vnd.ms-excel,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet,application/vnd.openxmlformats-officedocument.presentationml.presentation,application/vnd.ms-powerpoint,text/plain
and republish your survey.
Thanks,
Jody Zheng Liu