Not a bug, it models real life.
That's a very harsh statement you make there... Have you thought very thorough what you're saying? You say you are sure you know how politicians behave and you can predict political reactions! Maybe you should join a Think-tank on politics!
Maybe you didn't understand the situation completely. But since you talked about 'your' reality.. Let's use as example to point out the situation 4 countries from the real world.
We're having 4 countries. Russia, Belarus, Poland and the Ukraine.
All are friends. All have +(xxx) on diplomatic relations to another.
In this situation, Russia now declares war on Belarus. Launching already some nukes at them. Poland and the Ukraine, declare war on Russia. They are now (I checked it in my game) not friendly to Russia anymore and at war. All 3 against Russia. Now Russia also throws nuclear missiles on the Ukraine and Poland. All 3 are now under nuclear attack by Russian nuclear forces. Now the Ukraine and Poland decide to retaliate against Russia and launch nukes on Russia. Belarus still holding back it's nuclear missiles, trying to fight the war conventionally, not to unleash the full power of Russian nuclear forces. First cities of Ukraine are falling. Russia is invading the Ukraine massively. At the border with Belarus are now gathering massive amounts of Russian troops. Now they start to invade Belarus. Ukrainian cities are already overrun. Nukes flying from Russia on all 3 countries, and Ukraine and Poland nuking back.
First now, now Belarus is on the verge of overrun as well by the massive amounts of Russian troops crossing the border, Belarus decides to launch a small nuclear strike on those troops. No cities are hit. Belarus picks the spot to nuke very carefully, trying to hit only the Russian armed forces, not any civilians. While Ukraine and Poland nuked cities, Russia nuked cities as well.
Now the question, how can you be so sure, the the launch of 5 nuclear missiles from Belarus on Russian forces crossing the border, will have a negative impact on the relation between Ukraine-Poland versus Belarus? You say that's your real life.
IMHO, I must say, I don't know. And I strongly believe you can't be sure of what you're saying that it's reality that Ukraine and Poland will start to dislike Belarus because it's trying to hold off Russian armed forces by nuking them at the border. But you seem to be 100% sure about it. Maybe you are, like I said, I don't know. So please enlighten me about your logic to make me understand.
But if you however reconsider your stance, saying that indeed politics are usually hard to predict, and you can't be sure in such case whether relations will remain as they are or become a negative influence from such events, or maybe even stronger political ties because Belarus is now joining the efforts to stop the invasion and hold off the enemy, even if it means sacrificing Belorussian territory to the 'inhabitable due to nuclear fall-out' like Poland and Ukraine already did, and Russia as well...
Then I consider this a bug in the game. Because the game says it's 100% it will have a negative impact if Belarus in above situation will nuke the enemies forces crossing it's border. And I strongly believe you can not know this! It might, under circumstances, it might not.. Depending on general public opinion, how the newspapers bring the story.. maybe people in Poland and Ukraine want revenge because Russia nuked their countries, forcing their politicians to support Belarus now even more and think it's the right of Belarus to retaliate? How do you know so sure?
Any way, in the game I checked it while debugging with VS2003. Indeed the negative points come from nuking the invading forces at my border. Nuking the enemies cities and killing his civilians my 2 allies like and enjoy! But when I nuke forces crossing my border, spreading fallout at the border also at my side, then they get a negative impact from it.
And I truly believe, you can not in real life be so sure such an event will cause a negative impact.
I'm rather expect the opposite, talking about real life, after Russia would have launched Nuclear Missiles on Ukraine, Belarus and Poland the rest of Europe would hate Russia. And politicians in Europe wouldn't blame either Poland or Belarus for retaliating nor would they get negative impact between another for retaliating. I'm sure some people would protest, sure they always protest. But the political relations probably even bring those countries more together.