Sorry for the delay in our response! This is a tough one. It looks like other users have experienced this issue when updating iOS on their device. Some reasons I found this may occur:
If you had multiple accounts on your device and for some reason you are no longer signed into the email account they were synced to.
If sync for Notes got turned off inadvertently in the iCloud settings sometime on or around 7/20.
I found information that indicates the data (if present in the backup you made before the update) may be recoverable using 3rd-party software. Were the notes there just prior to the update & backup? If so, that may be a way to go. We do not have any specific software to recommend, and they can be quite expensive. One we came across was called dr.fone Recover iOS. Please note we have not tested or verified this software.
I found a few other links that may help you out. Otherwise, you may try reaching out to Apple again. Perhaps the next person who helps you will have seen this type of issue before and know some other suggestions.