🙂 it is not flawed, although it could be better. But it works as intended. If your refinery does not pull cargo from the mine from which you are delivering, it means your transportation is wrong.
The time is of the essence here, if your transportation is not able to deliver the cargo in the fastest way, then it will prefer the walking route. And if I say the transportation time, you need to consider all the waiting times on stations, walking time to station and from the station.
I am only sad about the growth rate of the production, even if there is a demand, the factories and mines grow only by 1 unit per month. Which is ok when playing from scratch. But when you try to add new refinery to help with delivering goods of a fully topped up refinery, it is painfully slow to catch up a demand of 400+ units.
But there is one actual flaw. And that is the steel mill production chain. Oil and wood are 1 source unit to 1 goods unit. But steel mill is 2 source units to 1 target unit. Which makes it more profitable to run only steel chains as you have double profit from the 2 lines to feed the steel mill, than you would have from one line you feed oil and wood chains. So if I use oil and wood, it is mostly because I want, not because it is best.