Sounds more like a bug to me. In any other case when the tile is affected by 2-3 different cultures the culture with the highest percentage must win the tile! Now, as my units kill French units their culture is being decreased by the influence driven modcomp, while my culture increases. But instead of flipping to me the tile flips to neutral Belgians the moment Belgian culture overcomes the diminishing French culture! Since Belgians now have open borders with France and I don't -- I cannot attack the French without declaring the war on Benelux

!
The only consolation is that all the British anti-Air measure are utterly ineffective against the might of my beloved Luftwaffe. We are running out of Factory buildings we can destroy in London

, perhaps Portsmouth will be next target!