Maybe the result is correct, because MyClass cannot be assigned to all of IMyInterface<,> (such as IMyInterface<string, long>, for example), but it can be assigned to particular IMyInterface<MyClassModel, int>. This returns true: typeof( MyClass ).IsAssignableTo( typeof( IMyInterface<MyClassModel, int> ) ).
To check that MyClass implements the generic IMyInterface, try something like this:
bool result = typeof( MyClass )
.GetInterfaces( )
.Any( i => i.IsGenericType && i.GetGenericTypeDefinition( ) == typeof( IMyInterface<,> ) );
Are you sure that IsAssignableTo was available in .NET Core 3.1?