Most/all modern TVs seem to have a two-stage standby. When you switch off, it initially enters a standby state but still consuming significant power. After a period, maybe about 10-15 minutes, if you have not changed your mind, they enter a deeper sleep, consumption dropping to a fraction of a Watt.
I think the reason for the first stage is, if you do change your mind and power on again, it is less stressful on the TV’s electronics. It’s probably also quicker. Point is though, it is quite possible that USB power might be maintained in the first phase, then turned off for the deeper sleep that follows?
Only way to distinguish between the two states, on my Sony or Panasonic at least, is to connect to the mains via a plug in power meter, and then check at intervals until it drops to <1 Watt.