CO and NW do not have the same criteria for processing upgrades. On CO it does not matter if you are plat on NW or CO both are thrown in the same pot and processed by factors such as fare paid, date bought, etc. On NW it is all NW plats (regardless of class paid or date bought) before any CO plats are processed. Then all NW Golds, before any CO golds, etc, etc.

Another major difference between the two is in companion upgrades. On NW metal a NW Plat companion gets upgraded with the plat at the 5 day window. Day of at the gate, Gold and Plat companions are upgraded after all Elites (NW and CO) are upgraded if space is still available. On CO for day of upgrades at the airport the companion is put in the same order as the Plat/Gold and upgraded before any silver has a shot.

So from your description it sounds like it is working exactly how it was programed to work.