The rules are clear. I would write to UA asking for compensation
Conditions: Involuntary
• When an airline undergoes a schedule change or encounters an irregularity in accordance with the general
provisions of IATA Resolution 735D, the Star Alliance member carriers agree to grant airport control
(and thereby waive the endorsement requirement) for the following:
A. non-restricted tickets;
B. restricted tickets;
C. Frequent flyer redemption tickets. After schedule changes, rebookings are made with Star Allliance
carriers according to redemption seat availability.
• No endorsement required for any fare types when an airline encounters an irregular operation (within 24
hours of departure).