Hey hm1818,
It sounds like you've done some great troubleshooting to narrow the issue down so far. Next, we'd recommend checking to see if any Do Not Disturb or Focus settings are enabled that could be preventing the notifications from showing. This article provides more information about that: Use Focus on your iPhone or iPad - Apple Support
If that doesn't help, you may want to provide some additional information about the situation.
Is the issue only occurring with notifications from the Messages app or are other apps affected as well?
Since the issue began, have you been able to restart your iPhone? If not, we'd recommend trying that as that could help resolve the issue: Restart your iPhone - Apple Support
Thanks!