I can't answer. I thought from the subject that this thread would be about supply lines cancelling ~unexpectedly~.
I set up a Supply Line from the Slog to the nearby greenhouse settlement and of course my Workshop access tied into the Supply Web and I had 1,200 steel to craft at the new base. I started dropping turrets and then suddenly I had only 11 steel!?!? I wondered what was going on, backed up a few levels then went back and all of a sudden I had 1,000+ steel available again.... placed a few things and then it's back down to the steel on my person.
I finally realized that there was a Mister Gutsy just down the road that was whacking every provisioner that left The Slog heading that way to "establish" the supply line. A new settler apparently took up the supply route shortly, but then was cut down my the Mister Gutsy and I lost my connection to the supply web again.
I WOULD like to know the actual answer to your question, though, because I also created a giant interdependent web of supply lines ---- probalby fifty Provisioners running around the Commonwealth now. On my next run, I'll definitely limit my number of "links".