Losing Received Messages on Phone Startup

When my phone is initially turned on there are notifications displayed of messages that have been sent to the phone while the phone was off. After unlocking the phone, the messages are not in the Messages application and no longer displayed in the notifications section.

This issue does not happen every time the phone is off. Seems like when the phone is off for an extended period of time, i.e. for 8 hours while I'm sleeping, is when it happens the most often.

Why is this occurring? Is there a solution to get these messages that I get notifications for, but aren't actually getting stored/received by the phone?

Current Temporary Solutions:

1. Keep the phone on all the time
2. Set the phone to airplane mode before I turn it off so that it's in airplane mode on startup. Unlock phone before setting phone off of airplane mode.

Phone Specs:

iPhone 6S

iOS 11.2.6