I have the same problem but it's always been there, fitted an aluminium rad, still the same, fitted an aluminium block plate from the base of the intake behind the grill shaped down to the bottom of the rad to prevent spillage, not sure about the result of this because I did it recently after the cold weather started but it looks like it'll be the same so it's interesting to read about the air pressure build up in the engine bay, hadn't thought of that with all those louvres facing aft. I'll wait till warm weather and bear that in mind.
Simon 2002 4/4 1936 Austin 12 light 4 Ascot saloon 2023 AMG Mercedes C300e estate 2021 Audi A1 sport