SUMMARY
on a multi monitor setup with 2 screens, kwin places the mpv window in full screen mode on screen 0 (the primary screen) instead of screen 1

STEPS TO REPRODUCE
1. mpv --screen=1 --fs-screen=1 --fs video.mp4
2. starts the video on full screen on 0 instead of screen 1

OBSERVED RESULT
mpv --screen=1 --fs-screen=1 video.mp4

won't start the video in full screen but places the video on screen 1

EXPECTED RESULT
window gets places on screen 1

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
using for example fluxbox instead of kde/kwin, fixes the problem and the window gets places in full screen on screen 1, when the video is started with

mpv --screen=1 --fs-screen=1 --fs video.mp4