DC Motor Theory and Practice
The higher voltage caused the motor to run at a higher speed.

As your load was a propeller, whose torque and so current demand goes as speed squared, this resulted in a larger increase in current. As heating in the motor goes as current squared, this resulted in a much larger heating, which burnt out the motor. So if we take only the 14.8 nominal voltage, the current would rise by (14.8/12)^2 = 1.5, and the armature heating by (14.8/12)^4 = 2.3. At max voltage, it's much more.

Supplied with less than its rated voltage, the 24v motor will run slower than its rated speed. It's still possible to burn it out if the propeller load is high, and so it demands too high a current.

Given you're driving a prop load, it would be a good idea to have a speed control. It's probably more convenient to have the battery voltage above that of the motor and buck it down, as this gives you 100% speed control, and a bit of overrange if you need it. Using a battery voltage below the motor requires a buck-boost converter, a bit less efficient than a simple buck.