It’s not a game bug; that’s Steam’s DRM.

Funny how you got hit by that on an domestic train trip and I traveled abroad several times and not got that weird behaviour even once. I simply never use offline mode. On the plane I was in airplane mode and when not on the plane I was on hotel wifi, personal phone hotspot, or just not connected to any wifi. Steam also never just out of the blue validated my game data. Must be a problem on your end.