Does the white thumbnail show a cloud? Then Photos is waiting for access to iCloud after editing to sync the changes with iCloud.
Are you using iCloud Photo Library? You may be using it without knowing this, because the iOS 11 upgrade enabled iCloud Photo Library by default, with the option "Optimize Storage" on. With the option "Optimize Storage" on some originals of the photos may have been removed from the device, and your iPhone will need internet access to download the originals from iCloud, when you try to edit your photos.
Check, if iCloud Photo Library has been enabled. If yes, connect your iPhone to a network, so it can download the originals again. Then turn off "Optimize Storage" if you do not want this option.