Divisions disappearing

I had an entire division of 900 OAAB and 900 MRMB staged on my country border then it disappeared randomly. The division was automatically dismantled and all weapons deactivated. The ammunition which was at max capacity is gone and not in my stockpile. Now i have to wait months to reactive all those weapons and wasted hundreds of billions on ammunition. What on earth is the explanation for this and why is it in the game? I am heated.