It seems like you're encountering an issue with changing the wallpaper for individual chats on WhatsApp using your iPhone running iOS 15.4.1. If the procedure you mentioned isn't working as expected, you might want to try the following troubleshooting steps:
Update WhatsApp: Make sure you have the latest version of WhatsApp installed on your iPhone. Outdated versions could potentially have bugs or compatibility issues.
Restart WhatsApp: Sometimes, simply restarting the app can resolve minor glitches. Close WhatsApp completely from the app switcher and then reopen it.
Restart Your iPhone: A restart of your device can also help clear out any temporary issues that might be affecting WhatsApp's functionality.
Check App Permissions: Make sure WhatsApp has permission to access your photos. Go to "Settings" > "Privacy" > "Photos," then ensure WhatsApp is enabled.
Check Storage: Ensure that you have enough storage space available on your iPhone. Sometimes, insufficient storage can affect app performance.
Check Internet Connection: A stable and reliable internet connection is necessary for many app features to work properly. Make sure you're connected to a Wi-Fi or cellular network.
Clear Cache: If WhatsApp has a cache-clearing option, try using it. This can sometimes help resolve issues related to app performance.
Try a Different Image: Sometimes, specific images might have compatibility issues. Try selecting a different image to see if the problem persists.
Contact WhatsApp Support: If the issue still persists after trying the above steps, you can try reaching out to WhatsApp support again. Explain the problem in detail, including the steps you've taken to troubleshoot, so they can provide more specific assistance.
Wait for Updates: If the problem is a known issue, it might be resolved in a future update. Keep an eye out for updates to both iOS and WhatsApp.
If all else fails, you could consider waiting for further updates or using the global wallpaper feature until the issue is resolved. It's unfortunate that you haven't received helpful assistance from WhatsApp so far, but trying the steps above might help you get closer to resolving the problem on your own.