Thanks for asking the Apple Support Communities for help with iCloud Notes not syncing since upgrading the iOS. I am happy to check this out.
I recommend going to Settings > [Your name] > iCloud and turning the slider for Notes off.
This will stop Notes from syncing, but the Notes in iCloud will be saved and will re-sync when you turn this back on. Restart your device: Restart your iPhone, iPad, or iPod touch - Apple Support
Then, go ahead and turn the slider back on. Settings > [Your name] > iCloud. This will get everything re-syncing again. It may take time for everything to sync properly, especially if you have other information syncing with iCloud at the same time.
If you still have an issue, try on another wireless network, or using your cellular data. To use cellular only, turn Wi-Fi off in Settings > Wi-Fi. Be aware, this will use your cellular data until you re-enable Wi-Fi in Settings. Thanks and have a wonderful day.