This is from a sandbox save with a simple provide > request > depot loop. The depots ones with no priority are closest to the provider and requester stations and are being chosen first over the further but much higher priority ones. A train is sitting in the depot priority: 999999999 stop, and wont ...

My trains are not prioritizing them when returning to depots, and the trains in those depots are not priortized first on deliveries. I even have made the priority a million with no noticable change.