Flickering frames when reversing video?

Hey guys, I've been trying to export a video I've produced but unfortunately one of the sections keeps having flickering frames.

https://drive.google.com/file/d/1lpCtumo3Bapy5pG4rjOcpY3pMe5D354h/view?usp=sharing

This is the best I was able to get the video (only 1 frame of flicker) but it still ruins the video.

This flicker occurs on the 'curtain closing' green screen overlay. The cause appears to be me reversing the video -- the video is ordinarily curtains opening. When the video isn't reversed, there isn't this visual glitch, but when it's reversed it is there.

These are the effects on the clip:

The posterize time seems to have helped in reducing the flicker, and in an export of a previous project it actually seemed to completely take it away, but for some reason in this project I don't seem to be able to completely remove the flicker no matter what I do.

I've tried fiddling with the anti-flicker filter, changing the posterize time effect, and using different export settings but nothing has worked.

Does anyone have a solution?