In another article, I read a US citizen being detained despite showing a copy in his phone: https://archive.is/0WXZR

Edit: actually I'm not sure if he got the chance to show the copy, that info seems ambiguous:

The federal agents who detained Mubashir refused his repeated attempts to show them a copy of his passport on his phone or provide his name and date of birth to prove his citizenship, he said. Instead, they insisted he allow them to take a photo of him to make the verification, according to Mubashir.