I have this bug aswell. The first time I played through the mission I had no artillery and didn't know I was supposed to have it, tried taking the base a few times on my own succeeded once only for one of my men to die afterwards meaning the whole mission was a loss, it's daft that a war would be called off just because one soldier dies.
Then I found out I was supposed to have artillery so restarted the mission and there it was. Played through to the main camp again, used the armor artillery which had no effect on the tank and bmp. So reverted to try something different but my artillery didn't return when I reverted. Very annoying.