The report mentions several issues plaguing the new iPhone, which analysts have suggested may face delays: Problems with wireless charging, which would allow users to charge the battery without having to plug the phone in, and problems with a 3D sensor for facial recognition, which may be used to help users unlock the phone.
Samsung includes both wireless charging and support for face unlock on its new Galaxy S8, but Apple hasn't worked with either technology before. Fast Company said there's a "sense of panic" among the team working to build the new iPhone and a fear it might not meet tight deadlines.
Apple typically launches new iPhones in September, but some analysts have suggested that the flagship high-end model might not launch until October or November.
CNBC reached out to Apple for comment, but a spokesperson was not immediately available.
CORRECTION: This article has been updated to reflect when a new iPhone typically launches.