From: París, FRANCE
I am Harrybanana's opponent in the game mentioned in the original post.
I can confirm the game was started under the latest official version, v1.00.073
This version was issued July 15th long after this bug was reported and corrected (in May).
I just ran the hotseat test and the soviet corps went from gun:2 to gun:1 after Germany DOWed Soviet Union.
Then I checked the _events.txt file from the Europe 1939 folder.
The values were wrong : stats=20,3,1,2,0,0,0,0,0,0,0,1,3,5,1,60,180,0,0
I edited the file and set the values to : stats=20,3,2,2,0,0,0,0,0,0,0,1,3,5,1,60,180,0,0
Then I did the hotseat test again and got the exact same result : corps guns dropped to 1 after German DOWed Soviet Union.
Looks like something is wrong here, and I don't have a clue how to deal with it.