The times I have encountered this, it had to do with how the Row Level Security was set up in the Power BI reports and how the users were authenticating in via the iPad. Since iPads do not have integrated AAD, they use different methods for securing the connection to anything MSFT-related. 

To test this out, create a blank report with 2 measures, review the results, then review the Row Level Security filter context.

Username = USERNAME()

Userprincipalname = USERPRINCIPALNAME()

With the issue you described, I would expect that you would see something like this:

Username = [email protected]

Userprincipalname = [email protected]

Then when you look at how RLS is set up, the values will be filtered to the opposites of what are coming through.

ie:

RLS is looking for:

Userprincipalname = [email protected]

Where the Userprincipal being passed through the iPad/Mac connection does not match. 

I hope this helps. 

Good luck.