Qi Charger broken magnetic backing plate
These plates are ferrite sheets. They are used to conduct the magnetic field, to increase the efficiency of the energy transfer. As it conducts the magnetic field, it will also reduce the magnetic field behind the ferrite sheet, which is quite important on the receiver side (less electromagnetic interference, less eddy current losses, less heat).
A broken sheet will decrease the positive effect of the ferrite sheet. This means the charger will lose efficiency. This could potentially lead to overheating of the charger. Depending on the implementation it will catch fire or turn down the power level to prevent overheating, resulting in a longer charge time.
I'd get it replaced if you want to use it for a prolonged time, if you use it without having it in sight, I'd definitely get it replaced.