https://www.flyertalk.com/forum/united-airlines-mileageplus/1758453-app-boarding-pass-does-not-update-wrong-gate-5.html#
Yes, this happened last week to me. I flew ORD-DEN and DEN-ORD same day turn. The return disappeared from the wallet. I generally always take a picture of my online boarding pass as I've had this problem occur before. I do use iPhone.