How does a powerbank detect a connected USB device?
Usb power banks tend to detect in two ways. One is by the pull up and pull down resistance on the data lines. As the RPI power input has no data lines connected, it won't detect a change of state. The other is by how much current is pulled once the plug is connected. If the RPi doesn't pull enough for the bank, the bank will think it's done and goes to sleep.