Hello DarylGS,
Let's use the steps below to update your iPhone to the latest iOS 16.7 as it contains important bug fixes:
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
You can also find those steps in this resource: Update your iPhone or iPad - Apple Support
If you have already updated, let's restart: Restart your iPhone - Apple Support
1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
Updating and restarting can help to further isolate a potential software conflict.
Should that still persist, do you have any other calendar accounts set up? If so, are they exhibiting the same behavior?
Also, are you facing notification difficulties with any other apps or features?
Let us know, and we'll keep moving forward from there.