Artifacts/Gamma Shift on Letterbox When Exporting HD MP4 from HD ProRes (Help!)

I'm noticing some very odd artifiacting/gamma-shifting on the lower black matte of my HD MP4s that I'm exporting from Premiere Pro (latest version). Has anyone experienced this? 

My source is a ProRes 422HQ, also HD, without any of these issues. Basically, it's not constant throughout, there's just a few shots from my film with a split screen where the images "slam down" into place, and when they do, it's like they're triggering some "flashing" on the bottom black matte of my 2.40 image (originally shot in 4K). It's mostly the black flashing a bit brighter but I'm also noticing a vertical line or two that flashes as well.

When I export from other programs such as Resolve, I'm not seeing it. Has anyone found a workaround for this?