The problem is that you didn't book the ticket through the airline that is flying the flight, but through a third party, and airline CoCs very explicitly say "we don't guarantee the flight times available at time of purchase as part of this contract with you". This, plus this being an award combines to reduce your leverage considerably when you've involved third parties that make schedule changes (US can't open their own inventory to help you, it has to beg partners for help, or you have to ferret out inventory on your own).

Tickets paid with cash (especially in premium cabins) are generally more flexible than award tickets, especially since a refund to original form of payment, your last resort when schedule changes mess you up, gives you cash to use for another cash ticket, instead of miles which may not be usable for another award. Also, airlines are more willing to endorse a cash ticket to another carrier than an award ticket, since they may not take a bath on that the same way they would on an award ticket; US isn't reimbursing a partner very much, and having to buy a customer a premium cabin seat on a different airline for customer service reasons might be rather expensive.

This is just the way it is. US premium cabin awards are great deals, but there's a flexibility tradeoff when you use them for travel in premium cabins as opposed to paying for a very flexible premium class ticket with cash. It's just not the same as dropping $5k or more on a plane ticket. You get a good deal, but to some extent you also get what you pay for; you elect to pay less and get less flexibility.