Adding a data point. We booked ANC-SEA-MSP and MSP-SEA-ANC. Alaska cancelled both of our flights between SEA and MSP, rebooked us on Delta both ways. Alaska mileage activity posted for our ANC <-> SEA flights but I needed to call to request Original Routing Credit for the SEA <-> MSP legs. Customer care asked for PNRs and had the appropriate EQM, RDM, and segments posted to our accounts within 10 minutes.

I really don't care about the Delta side but was curious and am posting here for others to reference. On the Delta page to request mileage credit when I enter the ticket number it shows the flights but that we would receive nothing across the board.