I'm pretty sure, the UA upgrades into P. That the SAS app shows the C as booking class is probably a fragment of the AC ticket in-between. I guess UA upgraded the legs to P, and did send AC the segment was upgraded into business class which maps to C on AC ticket stock by default if not specified otherwise. But for your milage earning, the UA booking class applies, not what AC or SK thinks it was. Then you did get the milage for P, 100% miles or 600 points minimum.