Servo RPM dropping without any physical distance change. Drone flips when trying to fly

Hello, I am using a Begalbone black with BLheli 30A ESCs. I have calibrated my ESCs, radio, compass, and accelerometer. I am running the 4.0.11 copter firmware. Is it normal for the PWM signal to drop after the drone is armed and provided with let’s say 50% throttle? The issue is that for a few seconds, the PWM signal remains constant. Afterward, the PWM signal starts to drop consistently, either on one of the motors or in three motors together. Is this normal? I have done a test flight and the drone tends to flip on the left side. I ensured the props were correctly placed. Can I please get some suggestions on what parameter or hardware to look for debugging?