Hi Chris,
Thanks for sharing your experience on this forum and we will try our best to help you.
According to your description, generally, if you set the workbook permission to share the workbook with everyone, including people who aren’t part of your organization, once you embed this Excel file
on your website by setting the SRC attribute of an <iframe> element on the web page, everyone will see the file in your SharePoint without login.
As you need to login first when access this embed file using the Safari browser on iPhone, to narrow down this issue, please try to change another browser like Chrome to check if this issue related
to a specific browser.
Besides, could you please the detailed steps to embed an Excel file to your website when you have time?
We can understand it could cost you some time, but it is necessary to help you more efficiently, your understanding will be highly appreciated!
Anyway, I ‘ll continue to help you, looking forward to your reply,
Best Regards,
Higgins