Hello,
A Buck converter with a MPPT control strategy would be focused on changing the current drawn from the solar PV panels dynamically in order to find the maximum power point (V*I).
Typically, these converters are connected to loads like voltage sources (typically batteries or inverters), not resistors. If a battery is connected to the output of this buck converter, the output current will change according to the available power. The same for a properly controlled inverter injecting current to the grid.
In the other hand, if you connect a resistor, you will only be able to track the maximum power point if you allow a variable output voltage. If you fix output voltage it is not possible to reach the maximum power.
If the panels are used for a stand-alone system, MPPT only makes sense if batteries are used. If no batteries are available, control the buck with a fixed voltage output and expect not using all the power available and power outages when there is not enough sun!