Remember that everything in the CO world works off of batch jobs or sweeps.

If you flew a segment and then get rerouted on another segment, if the agent deletes the already flown segment from your itin, the miles will not post.

Old UA used to post based on the gate info (if your BP scanned, your miles posted).

CO/new UA posts based on what is marked in your PNR as having been flown when they get around to sweeping the PNR for mileage credit.

It seems lame, but the upside of this design is that you can change your mileage crediting option after flying, but before posting.