Oh yes, I ran also into this.
When you edit the service file, you need a line ExexStart= before the actual one, so it looks like this.
ExexStart=
ExecStart=/opt/mjpg-streamer/mjpg_streamer -i 'input_uvc.so -ex 5 -d /dev/video0' -o 'output_http.so -p 8082 -n'
See: https://dietpi.com/forum/t/papermc-service-failed-with-result-exit-code/5882/3