The Aug 13 LIS-IAD flight was cancelled and replaced by UA 3027 on Aug 14. I don't see any cancellations on UA942 IAD-LIS.

Is this a coincidence? Probably. I would personally suspect maintenance as the cancellation reason as LIS is an outstation where they can't just swap in another plane. The 767s are also older and a bit more susceptible to maintenance issues I think.