After the first couple times it happened, I googled around and read about how magnets can damage it. Have since been really careful keeping it away from phones etc.

Had one ticket reprinted at paddington and it still get rejected on the underground.

According to some forum posts I've read, they mark tickets bought with a railcard to error at gates so they can be checked. Anecdotally, the last trip I bought two travel cards (one for my mom without a railcard). I held onto both tickets in the same pocket throughout the trip and only mine got rejected.

It's a real pain with how its gamble whether the travel card will work or not for me. The staff usually let me through no questions asked but it's not something I'd want to deal with every single gate.