Driving MOSFET with 26kHz PWM from NE555, MOSFET not opening completly
First of all I would strongly recommend you to add a parallel diode to the motor with its anode connected to MOSFET's drain and with its cathode to +12V. This diode should be schotky (1N5819) or fast recovery silicon (FRxxx, HERxxx, SFxx). This is done to prevent the inductive voltage overshoot of motor's winding right after MOSFET is turned off.

Second - your parallel capacitor 100nF is too big for PWM control. Each cycle the transistor is turned on a large amount of current will pass through this cap and heat up the transistor. A value of 1nF to 10nF maximum would be OK. If there is plenty emitted noise due to motor brushes (this is the main function of this cap) you could place a resistor in series with the capacitor - something like a 10-47 ohm. This way the stress on the MOSFET will be reduced significantly while the capacitor still does its job with motor brushes.

Measuring 8.7V at the drain makes me think that there are significant voltage overshoot after each turn off and thus the average value is high. Measure that again after placing the diode.