I think UA is not the single one to blame. *A is such a huge network with more than 20 members, and each members have their own computer system. It's tremendous work to connect the system with all their partner, so having some incompatibility is inevitable. This one, I think it's on OZ's end, as you can see UA confirmed the seat with OZ, and ticketed the reservation. It is OZ's system messed up the reservation and cancelled it. Furthermore, I have several reservation booked through united with partners like ANA and Air China, never had any problem like this.