Just newer iPhones??
Hmmm.... This may explain why wife's older Sprint iPhone 5S had problems connecting yesterday. I wonder if the problem is more widespread then this initial tweet suggests.
Sprint subscribers using recent model iPhones have been reporting cellular connectivity problems following the installation of Apple's iOS 12.1.3 security update, which was released on Tuesday. Sprint support acknowledged the incoming complaints via Twitter. "Some Sprint customers with iPhone X, iPhone XS and iPhone XR are …
If she gets a carrier update from Sprint and the problems go away, then maybe it is more widespread. If you check the rev number of the carrier 'blob' before/after an iOS update you can see if the carrier had an update that tagged along. They're responsible for testing iOS updates on their network and issuing updates to the carrier settings if required, looks like Sprint screwed up.
There are many possibilities - the obvious ones are ship's anchors in shallower waters, earthquakes, currents dragging the cable around, curious whales or sharks nibbling on them (yes, it does happen) and some deep sea shrimp that apparently take a fancy to the outer covering (some type of plastic) of the deep sea sections, thus letting in water which trips the power feeding to the amplifiers.