Answer
Potential energy gets converted into kinetic energy and heat, there is no violation of the energy conservation principle.
Work Step by Step
Let's assume that the bicycle is at rest on the top edge of the slope.
A component of its gravity is along the slope, hence, it accelerates under the force.
As it goes down, it gains kinetic energy because of acceleration and loses potential energy. Kinetic energy and the frictional losses come from the lost potential energy
