Venue networks fail. Your gate still has to let people in.
A signed list of tickets
An approved device downloads a signed list of the tickets for that event. The list only works on that device, and the device holds no signing key. It checks each scan against the list.
The server always wins
Scans wait on the device and go to the server when you reconnect. A scan the device rejected can never become an admission, and two scans of one ticket end as a single entry.