(Beta Version) The Rendered Results of 3.0 Rotobrushing do not follow the input I gave it

It almost always spills over into some other area, this is a major bug and I really hope you guys have spotted this.  Figured I'd post just incase, but this happens almost all the time. 

I follow the whole video through, rotobrushing it perfectly, it even previews perfectly in the timeline when I hit play, but when it's rendered it fails and spills over into other areas, meaning that completely irrelavent areas I never told it to mask somehow become masked in the render...

I'm using 4k footage, rendering in .mov's, input footage is .mp4's, that's all the info I can give, and I can't be bothered to give anymore sorry, thanks.