I just wanted to transfer all 20 siege machines to an alt in walled city.
I was astonished because when my alt had the 20 machines, in Questors backpack there were 5 siege machines remaining.
I logged both chars and the situation remained.
So 5 machines came out of the blue...

Its definitely a bug. If wanted, I will test if its reproducable.
Others may do the same and report their results here.