I've been watching the C award space on UA to several destinations (Budapest, Prague, Florence, Munich) and returns for May 2022. The availability was pretty good even with the direct flight from MUC to ORD open for most of the month. I scheduled my outbound easily for 2 seats. I began to monitor the return flights for after Memorial Day, and the availability was still almost unbelievable. Maybe it was? On June 30 all of the UA flights disappeared from the May calendar, except for May 18th. Many of those flights had 5 or 6 seats open on most days, including weekends. My outbound tickets are still in my itin, so those seats were real.

What would cause the entire inventory to disappear overnight?