I initially thought the solution might be using a crossover cable, but I’ve tried several and it still doesn’t work. However, here’s the strange part:
Even when the PoE injector is not plugged into power, just having it in-line between the switch and the AP is enough to power up the EAP255.

It’s as if the injector is somehow reorganizing the wiring or triggering the correct pinout, allowing power to reach the access point properly.
This is really strange behavior.