I had the same bug, and although I had some trouble reproducing it (because of some color blindness), I definitely took possesion of England and Castille this way in the 1440 scenario.
Long story short :
- played with France;
- in a alliance with Bourgogne and Nevers;
- Castille (already spreading across actual Spain territory) began the war with me (so I was the main defender);
- had some trouble but managed to invade their territory (Bourgogne also invaded some territories)
- during the peace treaty, I was supposed to choose my territories, but