Update 2/18/21: This appears to be working again according to US Credit Card Guide & FT datapoints.
Beginning January 14th, United Airlines Travel Bank purchases no longer work to trigger the American Express airline incidental credit, according to reader ChurningExperiences and various data points on Flyertalk.
They switched the category of the purchase from “special service ticket”, which was considered by the AmEx system as an incidental charge, to “gift certificate” which is not. Purchases made January 14th and beyond will not get credited. Purchases made on the 13th an earlier were credited.
Major bummer, lots of people were using this. Hopefully everyone used up their airline incidental credits immediately in early January when it was still working.