When playing Weave 20 with a group of 4, we barely got enough spawns to complete the first part. I remember similar issues during the private beta on the same map (Pasterole / Against the Grain) as well.
Back then the issue occurred every time I was playing with a group, but did not appear when I played those maps solo.
The main difference when playing with a group vs. solo was, that when playing alone, I would make sure to only ever trigger one ambush spawn at a time. But as a group we usually keep moving forward until there are enough enemies to slow us down.