Example: https://imgur.com/56vjNTq
There is over 132k of iron plates in buffer depot, however none of the drones will ever pick up from it. They always pick from supply depot.
My base is to the right side.

They always pick up from whichever depot is closest that has enough items, are you sure they aren't taking from the buffer depot?

yep I am sure in that scenario
if buffer depot is close to supply depot it seems to always priority supply depot

As was explained in another topic, resources were already reserved by bots on the way (road is quite long)
Still it would be nice for buffer depots to have priority for resources - so we can actually set up buffer depots that do all long way hauling of resources. Probably not an easy task. For now I will split up my long range roads from main base.