I guess it has to do nothing with the booking class. The linked thread had a different background (invol rebooking).
Here, the problem is that the flight was OP by OO as UA Express:
Code:
[KVS Availability Tool 7.3.3/Diamond - Operational Info: UA 5424/06 Feb 2014]
Planned Flight Info
Segment Departure Arrival Duration
------- --------- --------- --------
SEA-PDX 19:05/Thu 19:53/Thu 00:48
Segment Flight Note
------- --------------------------------------
SEA-PDX AIRCRAFT OWNER SKYWEST AIRLINES
SEA-PDX OPERATED BY SKYWEST DBA UNITED EXPRESS
As OO is no *A partner, no A3 miles for this flight according *A rules (operating carrier). This is not correct, as this is no codeshare flight, it's a wetlease contract between UA and OO. Flight should be treated the same as UA metal flights...
That's a common problem with UA Express flights. There have been same reports with USDM with collecting or spending miles on this flights...