Maybe it is time to repost United's policy
Connecting flights
For most trips that include a connection, your checked bags will follow you to the last stop on your ticket.
You'll need to go to baggage claim and re-check your bags if:- You have a layover
- You make a connection that involves an overnight stay
- Your connecting flight departs more than 12 hours after you arrive at the airport
- You’re connecting to a trip on a separate ticket and the flight is not with a Star Alliance™ partner airline
- You’re traveling internationally but connecting to a domestic flight in your destination country
Usually, we keep checked bags on the same flight as the traveler. If we can't, we send your bags on the next flight with available space.
Plus there are reports UA may for higher elites interline with separate tickets to non-* carriers (if there is an interline agreement).
Note in all cases,
the operating carrier and UA must have a bag interlining agreement (co-shares does not change that)