I recently started playing on another server, and this problem is even more exaggerated when you first start the game, but it arises occasionally later. Once you level a decent amount of versatile Heroes, you can usually overcome most obstacles with strategic play, but it is true that sometimes you face a line that is simply unbeatable. And I agree that this is extra annoying when it happens early in the Expo, preventing you from getting most of your gold and scraps that day.
I contacted support about this, and the response was that they are aware of the issue but, because of the way lineups are generated for expo (based off your fight pit), this sometimes happens. I would not expect a solution to this issue anytime soon, so perhaps don't go too far up in pit unless you think the rewards outweigh sometimes losing expo. I would focus on leveling your heroes and getting more heroes, and soon it will just never be a problem for you.