The exclamation point can occur, if you have iCloud Photo Library enabled on your iPhone or iPad.
You will see it, if Photos cannot download the original image files from iCloud, because it cannot connect to a Wi-Fi or does not have a cellular connection.
Make sure, the iPhone is fully charged and connected to Wi-Fi, then try again to view the photo enlarged.
Also, disable Low Power Mode, if it has been enabled.
Go to Settings > Battery, then turn off Low Power Mode.