Thanks for the report! I'll need to figure out a way to have Game Maker detect if you're alt tabbed. It's supposed to only be rendering what's on screen but if nothing is then everything despawns.
Also I love this game but getting an S rank on level 6 and 7 is so difficult, especially with the targetting difficulties of level 7. Very fun tho. Keep up the good work!
It is indeed very difficult right now, but you'll be able to come back to those levels later on with upgrades!