It can definitely be a pain trying to reassign a Provisioner, equipping them with a weapon renamed according to their route can help identification.
Learning their routes is a plus, they'll tend to follow the roads like any caravan, naming them according to route will help with that.
Designating centrally located settlements as hubs for supply lines eases the confusion, and allows other settlements to focus on resources; for example I have over 20 settlements, all but one are connected to 2 main hubs(Oberland Station and Taffington Boathouse). Red Rocket is the outlier, it's linked to Abernathy's Farm, which is linked to OS.
Never been able to find the Provisioner for that route, or I'd reassign him to OS. I have sat and waited for a full day repeatedly, yet he never shows up, in either settlement.