OLED TV does not turn completely off when turning the TV off with remote.
This is normal behaviour.
The OLED TV has to perform a callibration every now and then. This is executed when the TV is turned off with the remote control or the local key.
In this situation the Screen will go dark, but the logo light will stay on till the callibration is finished. This can take up to 15 minutes.
Warning : Never turn off a OLED TV by switching off the mains. In this case the OLED TV will never perform a callibration.