I have done some more purely empirical experiments on this. I personally managed to compile my previous 5.3 project in 5.4 by setting:
CppStandard = CppStandardVersion.Cpp20;
in .Build.cs
This was not needed when creating a project directly in UE5.4. I even ported my whole 5.3 C++ Source to the project created in UE5.4 and was able to compile it without the need for this addition in the Build.cs file.
Despite fixing this in a nicer way by avoiding all source code modifications, I still have no clue what’s going on
Hope this helps someone.