Instantly deleted AI army

Game Number 4,167,370

It's day 9 on Europe Clash of Nations Map (Sorry I don't know the map number)

Bug Description:

It's my first game and I'm loving it so far, the AI's Communist Russia and the Russian Empire declared war and invaded me at the same time, the Russian Empire moved troops into northern Finland (see image) with 3 stacks, the first stack (which is gone now) consisted of 1 unit of inf and 1 unit of anti air, I told my garrison to attack the unit and it was a few mins away from doing so and I gave the same order to the light tank behind it and then poof. 2 Units disappeared with no report or nothing figured I should report it.