A quick search shows that both Frontier and Southwest currently offer non-stop flight between Milwaukee and Orlando. "Direct" flight means it stops somewhere. Even when covid wasn't an issue, non-stop flights tended to be between airline hub cities and major destinations. None of us has any way to know which flights might be added back to the airline schedules once covid is behind us. With fewer people traveling due to covid, airlines will offer less frequent flights or eliminate certain cities in order to balance supply with demand.
If you have to fly United due to an expiring voucher, I would look for the best available options based on your preferred travel times even if that means stopping somewhere along the way.