I presume this was a domestic upgrade using points.

I have a couple of these and the booking class (U) is show.

You may have to click the [+]More flight info toggle to see it. It certainly works on each of my upgraded flights. (Don't worry if you see "Upgrade status: Declined" this is a bug. As long as you see "Status: Confirmed" above that and "Class: Business (U)" below it.

www.Checkmytrip.com will show the cabin class (as in Business), but not the booking class (as in U).