Not sure if this is a bug, but I found it alarming.

Booking a one-way ticket EWR-DEN, I wanted to book a FC ticket and tried two options. Numbers are estimates.

$501 Main Cabin (1700 MQMs)
$750 First Class (2500 MQMs)

- Select a Main Cabin seat. I was offered a $249 upgrade or 24,900 miles. If I select Miles, I'm only given 1700 MQMs. If I do it with dollars, I get the full 2500~
- Select a First Class seat and use PWM and choose $250 - 25,000 miles. Cost is now 1 dollar less for 100 miles additionally redeemed and I get the full 2500 MQMs

Seems weird.