Rotoscope glitches for no apparent reason

imagine i'm rotoscoping airplane in video of plane flying through sky. , i'm overlaying onto background video of solar system.
even though have checked every single frame , made sure alpha boundary around plane, when export, .avi shows quick flashing of frames large portion of blue sky being overlayed too, though i've checked every single frame , sky rotoscoped out.

 

has had problem before? perhaps encoder problem?

 

thank you!

did use roto brush rotoscoping? if so, did freeze roto brush when done tweaking it?

 

if did freeze or if didn't use roto brush, need the standard set of information.

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