Why are my trucks not refueling?

Hello,

I have a pretty basic setup of 3 trucks getting copper, iron and coal for my phase 2 factory. The issue at hand is that my copper and iron truck just seem to forget to refuel?

This happens after a decent time after i manually refill them and I usually dont put a full stack of coal in them so I can see that they refuel properly and they do. But then again, after a while they run out of fuel. Why is this?

(They have fuel inside their truck stations that get autofilled)