When running steep inclines, your efficiency will be different than running on flat terrain. That means that you may not be able to sustain the same power running on steep inclines for extended duration or you will see your HR change while keeping power the same.

Runners that are strong uphill might see their HR drop while maintaining the same power and can run at a higher power going uphill. Runners focused on flat terrain will see their HR increase and will not be able to sustain the same power uphill that they could on flat terrain.