Same for me here. The problem is not only for Premiere Pro it affects Media Encore too.
I tested multiple codec in and out, and the problems stays the same.
I found a workaround in Premiere Pro, you can work in a timeline without maximum bit depth option check in the timeline settings. Then you can export this timeline with Premiere Pro (not with Media Encoder) with the maximum bit depth check and the export should be normal.
I work on a
iMac (Retina 5K, 27-inch, Late 2015)
Radeon RX 5700 XT 8 GB
with a clean Catalina install
This bug is really annoying! Most of my work must be 10 bit or more, Media Encoder 2020 is unusable for me right now.
I’ll stay on Premere Pro 2019 and Media Encoder 2019 for now.
Anybody reported this bug to Adobe yet?