I have a Pi4 with the standard raspberry-pi branded POE hat, and it works fine.
It is installed behind a monitor, which has USB ports. So I thought to add a USB-A to USB-C cable from monitor to Pi as a second power source. Again this worked fine, until I turned off the monitor.
My expectation was that the Pi would still get power from the POE hat. However it lost power immediately.
This is different to putting a UPS on the POE switch, or having a dual PSU POE switch. I'm specifically meaning to have a local power via USB, and a remote power via POE.
Is there a way to "dual-power" a POE-enabled Pi so that both sources work and can survive either dropping without warning?