Incapacitating Dahl Quest

Solution 1:

based on the ending I chose, is this intentional?

Correct. If you chose the other ending, then Dahl contacts you immediately after pressing the button. You then have a timer to get to the shuttle.

Massive spoilers:

Because using the Nullwave resolves everything immediately, there is no need for the simulation to continue. But after setting the self-destruct you still have the choice of whether and how to escape, and Alex wants to see what you do.