Maybe if just before you switch the scene, you do a loop on all zombies there, save their X, Y and health in an array.
After, when you go back in this scene, the array must be loaded on start of the layout.
Like this, the same zombies on same positions are still there. However, keep in mind that can be annoying if a zombie is camping near the door used by the player for coming back there.
You can check the Construct 3 plugin here: construct.net/make-games/addons/319/savestate
To be use at your own risk since it's made by the community and not Construct 3. So, community plugin always has a risk to break your game, but this addons got no angry devs so far.
Good luck :)