I have an iPhone X, and experienced the same thing after my iOS 14 update.
I reset and restored and thankfully now my phone is back to normal. Note that updating to iOS 14.3, resetting network/all settings, and toggling notification settings (in-app and in iOS settings) did nothing for me.
I’d recommend backing up (to iCloud and to a computer), factory resetting, and installing a fresh copy of iOS 14.3 and restoring from that backup.
I hope that works for you. I was so frustrated, particularly with the lack of support and solutions, and I hope this provides a definitive solution to everyone having this problem.