I think the issue is that there's no through-fare filed for MVY-BOS and then BOS-BWI. If you try to book a revenue ticket, the system will let you plate an end-on-end ticket (multiple separate fares on one ticket), but hiccups if you try to do this on points because I don't think you can combine different fare buckets on points. (see all the threads about trying to combine a Mint flight with a non-mint flight on one PNR where there's no thru fare filed).