This means that if you power a device using the splitter, and that device has a non-isolated electrical connection to something that’s independently powered, you may get a ground loop. This will usually manifest itself as the PoE injector shutting down, but may manifest itself as anything from shorting out components to starting a fire (extremely unlikely).
NOTE - This is not an issue if you have no non-isolated electrical connections going from the device attached to the splitter. So if you have a Raspberry Pi attached to the Ethernet cable on the splitter and being powered by the splitter you’re cool. But if you plug an HDMI cable from the RPi going into a TV while it’s being powered by the splitter, sparks may fly.
The solution is to either be careful, buy a proper isolated splitter, or use a DC-DC isolating converter.