Was playing survival mode. Time to time I had to switch to another task so I did it in 2 ways:
Battle was running and I just alt+tab to another application. After a while I turned back and had several waves of enemies same time. Even battle was running in background (wasn’t noticed before), or next wave is calculated regarding battle start time.
I paused the battle and alt+tab to another application. When returning and “unpausing” battle continued, but when all enemies were destroyed no more waves appeared for a long time. (I’ve waited 5 min).