Never had any "waves" or constantly spawning enemies in this mission.

I have been playing since day 1, so maybe it's a new addition to the game, as when I played I managed to take out all enemies that I encountered in the Sphere, all of whom were already spawned in the base by the time I left the building.

I thought it felt a bit empty for a base, but it was still fairly challenging.

Perhaps it is a new addition to make it feel more dangerous and just force you to leave the base as by this point you are still a newbie technically.