Efficiency is a key factor for high-power gyrotrons, particularly in future fusion power plants, where a large number of gyrotron units will be required for plasma heating. A single-stage depressed collector enhances efficiency from approximately 35% to 50% by applying a decelerating potential to the spent beam electrons before they enter the collector. Experimentally, a well-defined threshold exists for the applied deceleration potential, beyond which efficiency decreases and instabilities arise. In this work, we conduct a systematic study of electron beam behavior under very high decelerating potentials by simulating the beam within the complete gyrotron geometry. This approach enables a comprehensive investigation of reflected electron behavior and its impact on cavity interactions. The findings provide insights into the limitations of single-stage depressed collectors. Moreover, quantifying the level of reflected current that does not significantly affect gyrotron performance is a key parameter for the design and development of high-efficiency multi-stage depressed collectors.