The HTC One M8 for Sprint is receiving an OTA update which brings the 1.54.651.10 software version, along with a fix for the issue which causes the device to randomly loose access to its data connection.
The error is caused by multiple connection attempts on the voice and data network in low-coverage areas, which for Sprint is basically anywhere outside the major cities. Apparently some phones would then enter a state where they couldn't connect to any mobile networks at all.
Apparently it seems like only a few Sprint users are facing this issue, but even so, the update is marked "URGENT" on the OTA pop-up window. The update is around 56MB in size and is being rolling out in stages. Keep in mind that you may not receive the update notification immediately and it may take a couple of days to get to you. You dont have to rush to install the update, if you have not faced any issues on your Sprint HTC One M8, but it is always recommended that you have the latest software installed.
source - Sprint | Android Police