Deliberate, as some Hungarian forces chose to fight on even after Belgrade fell (unlike, say the Belgians and Brussels). I know it would be possible to withdraw some units and not others following the occupation of Belgrade, but Hungary isn't a major power and a random all-or-nothing seemed sensible at the time we designed it. By the time the capital's lost there probably won't be much of the Hungarian army left anyway, so it shouldn't be a decisive result either way. If someone wanted to alter it so that units surrender or not according to what they did historically it would be perfectly reasonable, but involve more work for little gain.