I think everyone encounters the issue of having dead units in the editor and live units in the game. And this is probably a bug. I can't say for sure how to fix it because I didn't figure out how the creatures became dead both in the editor and in the game. As an option, you can make the first (that is, actually 0) frame empty (but first, place a monster link there). And set the animation with a delay of -40 on this empty frame.

Added 23 minutes later
Urgent correction... if you read what I wrote above, forget it.

It's enough to simply place a monster on the map and set its Delay to -5. I just checked, it works both there and here. This is also how it's set up in my videos from the first campaign. It might only work with the 0th scene, but that's not certain.