I ask in order to exclude it is a bug:
It is possible to change monsters manually to other sprites, which are not provided by the “Insert action” functionality. I managed to make my monsters invisible that way and wondering, if I should use this for my game. However, this might be a bug in the engine or my game may changed unintentionally, if you change the tilesets or even crash. Is it safe to make my monsters invisible that way?
How can I do this? The only way I know so far is, to make the monsters inactive. But that way they will (and should!) vanish from the game, unable to fight against the hero. We cannot create creepy invisible monsters threaten our hero that way.
Do you know about “Final Fantasy Mystic Quest”? There is an “Ice Mirror” within the “Ice Pyramid”, which is able to make invisible monsters visible. What an idea! I liked it! With invisible monsters we could have things like that for our games on RPG Playground as well.