The behavior of the notification icon not changing right away after an app update is known, and is under investigation.
Although, at this point we are expecting the correct icon to be displayed after a restart.
If you are certain that after installing the version with the new icon and restarting the device notifications are still displaying the old icon, please submit a bug report about this.
Please find out from the users details about the issue: confirm they have installed the new version, confirm they have rebooted the device, and ask about their Home Screen setup. Whether they are using the default icon look, large size, dark mode or perhaps tinted icons.
Along with that info and also with the iOS version on these devices, if you could open a bug report, include sample code or models that reproduce the issue, that would be very helpful.
Bug Reporting: How and Why? has tips on creating a successful bug report.
If you are able to reproduce the issue on a device you have in hand, It would be very helpful if you could please go to https://developer.apple.com/bug-reporting/profiles-and-logs/ and follow the instructions for APNs for iOS to install a logging profile on your device. Then try to reproduce the issue by installing the new version on top of the old if you are still able to do that, and by sending a notification, then follow the instructions at the above link to create a sysdiagnose. And attach that to the Bug Report as well.
