I thought I might have just gone crazy (likely), but I've noticed that my vendingmachines (selling low-grade fuel, 1 sells a stack of 500 fuel for 100 scrap (cheap) and the other sells 125 fuel for 25 scrap(also cheap)) are sometimes not containing any scrap when it gets sold out.
Someone(❤ rhymes with Shamesy) mentioned later on that when people buy more than 10k worth of scrap in one go it falls behind the vending machine and despawns?
I'm guessing it's a problem with the stack sizes the machine can handle. As I've already reduced the input to max 60K fuel in the machine, I guess I have to reduce it further.
I hope this helps towards finding a solution.