What can be the cause for the servo to jitter continuously while powered?
Do you have a servo tester? It's a small cheap device with a knob, like this one https://www.amazon.com/RCmall-Digital-Consistency-Controler-Checker/dp/B01BY5LOZE 

It could be that you send such pulses from STM32, and that it's not a jitter rather your command pulse train. Keep in mind that servos accept voltage from 4V to 6V, while your STM32 works on 3.3V. It has 5V tolerant pins, but not all of them are 5V tolerant.

Show the schematics and link the program.