I think I have found the source of this issue...
In the options you can set the capacity of a fluid wagon I found theres serious issues with throughput when the capacity is at default so i raised it to 2500.
Now if all fluid wagons carried 2500 it wouldnt be an issue but for some reason fuel trucks seem to carry 6 times the fluid capacity amount which means they are able to carry 15000. the requester depot recipe is set to 5000 so the fuel truck arrives, dumps its load and the depot produces its recipe of 18000 items.
I can't find the bit of code that gives the fuel trucks a capacity bonus of 600%, anyone know where that is?
For now I'll just set drone fuel to 0 and play without fuel as i cant risk a delivery arriving and spawning 18k items out of nowhere