Things to try:
Make sure that the UUID of your device has definately been added to your Profile Provision as well as the new new developer certificate.
Ensure that the Certificate and Profile currently selected are the new ones in the ios settings in Unreal (importing doesn’t always automatically select the new one if you have existing profiles set).
I actually had the exact same error when I first tried, turned out that my certificate was still set to my previous one (if you loose track of which is which, look at the expiry dates).
Make sure you’re running a new enough version of macOS and xcode (the new format has supposedly been used for a few years so probably not that).
Make sure you are definitely loading the new .ipa and not the old one (I’ve done stupider things so always worth checking the basics)
Check out:
https://developer.apple.com/documentation/xcode/using-the-latest-code-signature-format
It has info on the macOS version you need, and also a terminal command to check the the new .ipa does indeed have the new signing version.
Let me know how you get on.