ratiometric hall effect sensors outputs at high RPM
You have a sensor with a 20kHz BW ( typical) and a flux signal from a 3mm magnet at a 30 mm radius (?) with loss of signal at 10kRPM or 167 Hz. 

The duty cycle of the pulse is less than 1.6% = 3mm/(30mm*2π) * 100% .   

This small duty cycle reduces the fundamental f signal by -33dB and spreads the bandwidth of the signal up to 1/1.6% or 62 times the fundamental , where there is a null in the spectrum.  

That is over 10kHz, so it means yes due to the small width of the magnet you will lose amplitude with RPM from the harmonics exceeding the sensor BW.


There may be some GBW sensitivity improvements that could be made, if you provide more details, but the fundamental design problem is the magnet width at high RPM.