As Savoy I declared a war of conquest for Pisa and invited my allies. Due to the computer's uncanny ability to react quickly one of my allies managed to move 1 regiment into the contested Province seconds before me and thus be the one to actually conquer it even of they're totally outmatched by my 20-regiment army that sieged along with them. Warscore is at 100%, all provinces are occupied but I can't claim Pisa in a peace deal since I'm not actually the one controlling it.
So, bug or feature?
So, bug or feature?