Not sure if this is a bug exactly or just how it works, but in other production lines I have not seen this happen.
I have no 737 NG on order but 150 Max frames starting in 2017. Any attempt to order additional NG frames gives me production slots after the current Max orders. In the A32X line in GW3, I was recently able to place an order for A320ceo's ahead of my 32X neo's, so I assume the issue in GW2 is a bug.
Edit: Never mind, this only happens when NG and MAX are ordered in the same order. Was able to top off NG fleet with 43 -800's with deliveries from April 15 to July 17, ahead of MAX launch.