You got lucky in the multiple solution (I was not as lucky as you and I got damaged by placing the "wrong" monster)
If having several valid solutions (but accepting only one) is a design choice, I'd say it's a dangerous one. It feels pretty unfair, and can be perceived as a bug, since the player is following the rules but being punished anyway.
I'm gonna be completely honest here, I haven't run into this case enough to even consider that it's possible to have more than one solution. Thank you both for putting it on my radar though. I'll have to consider what to do in these cases. I intuitively feel it's not fair to punish a player for this quirk of the puzzle generator.