Seems to be caused by an issue binding/modifying the default framebuffer.

https://www.youtube.com/watch?v=We0iXmlP3Po
ReplayRenderBug.txt

https://discordapp.com/channels/708304818829262910/709108200632483900/718545270299820082

I may have made a breakthrough on this issue. I removed the resource pack from the .mcpr file (use 7-zip to open the archive, rename the resourcepack folder to "ignore_resourcepack" and then load the replay). I've tried this so far with three replays that were giving me trouble and have had success each time. I will keep doing this to see if it is a fluke or a consistent solution. My resource pack changes (adds) a lot of custom item textures, but doesn't mess with block textures at all, but may be throwing things off. Still no idea why it only affects the render output.

Reported by Danbearpig#4035