Unwindable without cheese mission on the fountain map?

I ran into a “very difficult” mission in late early june on that awful map with the fountain in the center and no cover. Playing classic/ironman. I advanced slowly in my overwatch bubble but there was a doom blob of 8 muttons + floaters that were legit inside of each other in the center/back of the map that was impossible to not agro all at once and get slimed by. I was moving 1-2 tiles at a time and once one came into view 7 more would spill over out of the fog. I force quit the game as soon as i saw them, reloaded, and used 30 turns of the “destroy cover” ability on my mech to flatten the entire map before slowly using the same ability in an expanding grid out of site in the fog to try and pull them individually into my overwatch. This worked but felt super cheesy which i hate. Was this a bug or are there just a few poorly designed missions like this?