This seems strange. But here are some possible fixes.
# Check Apple Server Status [link|https://www.apple.com/in/support/systemstatus/|from here]. Make sure '''iCloud Notes''' is green.
# Restart your iPhone. (If this does not help, [link|https://www.igeeksblog.com/how-to-force-restart-iphone-ipad/|force restart].)
# Turn off Notes iCloud Sync and turn it on again. For this, open the iPhone '''Settings''' app > your '''name''' from the top > '''iCloud''' > toggle off '''Notes'''. Now, restart your iPhone and follow the same steps to turn it on. Everything should resync.
# Signing out of Apple ID and resigning may also help. For this, open the '''Settings''' app > your '''name''' from the top > '''Sign Out''' from the bottom.
# Make sure you and the person you are sharing are on the latest respective versions of iOS. (Sharing to iOS 12 or iOS 13 may have some glitches)
# Reset All Settings: Finally, if nothing helps, you may open the '''Settings''' app > '''General''' > '''Reset''' > '''Reset All Settings'''. (Before proceeding, take a backup to be on the safe side.)
Hope it helps.