It's a longshot guess but maybe Concur has the wrong airplane coded for this flight, and it keeps moving me to the E seat because that is (or used to be) a window seat on the MD aircraft? This is an A321neo.

Normally I like the window seats for the view and sleeping and I hate the bev cart assaults that happen in aisle seats so window is my preference, although on the ANC return I picked an aisle because I will want to get up a few times to stretch.

Adding that I did at least once recently upgrade with own funds without issue, although that was an RJ.