Howdy, the A save slot is currently the only one that works correctly in the demo. The issue has been fixed in the full game but I can't change the demo at the moment since I'm working on finishing the game.
Save slots B and C currently don't load battles correctly. Sorry about this bug, I'll try to see if I can fix it somehow but for now just use A.
"C:\Users\YOUR PC USER NAME\AppData\LocalLow\Kunstaat\Ant Keeping Simulator" is the folder you want to look for If you don't want to restart if playing on a non-A save. Delete the old save_0.sav file rename the save_1.sav (or save_2.sav) file to save_0.sav and now you should be able to keep playing with that save in slot A.