I booked an avios 2-4-1 redemption MAN-LHR-ORD || YVR-LHR-MAN earlier this week for me (Gold) and my wife (Blue) and MMB let me select 1A and 1C on the MAN legs and whatever I liked on the ORD and YVR legs with no problems. This was a single booking, not a merged / TCP booking so maybe its the merging that is the issue.