expo generates lines based on the team power of the line. So it isn't going to pull low power lines to players needing high power opponents. That's not an exclusion, that's the way it works. The issue arises when it tries to draft a low power line and finds some single toon whose power all by itself fits the request. The team-of-one power is in the right range for the player, but the single high-power toon isn't a realistic fight for someone who's team-power is at that value. Adding a provision to expo which chooses only 5 member teams for the pool, then drafts based on power per usual, is a sufficient fix imo.
On a sidebar, the issue does occur more in correlation with fp contests, it just shows up in expo the next day. I assume the server is culling lines from fp throughout the day to build a pool for tomorrow's expo's. If so, that process needs to exclude lines w fewer than 5 heroes.
Aside two, some high level players do see shockingly low teams in expo, but due to a different problem often referred to as the "wrap around" problem. Expo tries to creep difficult lines earlier in your sequence. Then when the algorithm tries to fill the last stage(s) with lines more powerful than what exists, it winds up w really easy ones instead. This occurs every day, but the weirdness from contests often causes it to shift further off base in my experience.