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Can't flip movie (Stimulus) with MovieStim3 when displaying on a second screen

As with the title, I tried using the .setFlipHoriz function but it does not work. The movie stimulus continued to be displayed in its original format instead of being flipped 180 degrees. Does anyone know why? I am currently using the latest Psychopy3 standalone version. Below is a snippet of the key sections of the code that I used:

EDIT: I just realised that this only occurs when the movie is being played on an extended (Experiment) screen but not on the PC original monitor itself. Would still appreciate if anyone has got an insight to this.

mov = visual.MovieStim3(window, filename='moviefile.mp4', units='pix', size=visible_px, pos=(0, center_y_off_dva), ori=0.0, flipVert=False, 
                 flipHoriz=False, color=(1.0, 1.0, 1.0), colorSpace='rgb', opacity=1.0, volume=0.0, 
                 name=None, loop=False, autoLog=None, depth=0.0)

if flip_screen == True:
     mov.setFlipHoriz(newVal=True, log=None)
mov.draw()
flip_time = window.flip()