Choice of heating element (power resistor)
I am going to use 25 V source supply


If your supply voltage source is 25 volts then you need to use this to calculate the resistance needed.

\$Power = \dfrac{volts^2}{resistance}\$

Resistance = \$25^2 / 21.25 = 29.4\$ ohms

The error you made was to assume that your power supply will always supply 2 amps irrespective. In reality if the voltage is specified then hanging a load on the output will take precisely the current needed as defined by ohms law i.e. 0.85 amps in your case.