I've noticed this for online boarding passes for domestic flights where the flight number also does an international leg, such as US 801, which I flew yesterday CLT-PHL. The flight # starts as GIG-CLT.

Assuming this is the common factor, it would seem a bug.

Edit to add: just noticed that every US flight I've been on in the past 2 weeks has had an intl segment to the flight #, so this is probably a red herring.