Any idea, why my spending space in The commonwealth of Belon on FB is 0 for at least five months now although I didn't purchase almost anything in this period (some 30 B monthly on contracts)?
Because you're auto buying function is buying suplies for you. And aparently it needed to buy a lot.
There is also a difference between military and non military spending. For your military your auto buying function can easily spend those 400B and not even break a sweat.
For your non military it can only do that if you had serious shortages and want high quality products.