High-Quality 4K Exports Looking VERY Pixelated

I've been having this issue for the last couple months with Premiere Pro CC (latest version).

It'll export 1080p files without any issues, but as soon as I feed it 100mbps+ 4K footage it spits out some heavily compressed files.

The screenshot below was exported at CBR 150mbps (source was 150mbps as well). Match source, maximum rendering quality and maximum rendering depth were all checked. However, the file Premiere Pro ends up exporting is 19mbps. This happens in both Premiere Pro and Media Encoder. It doesn't matter the video file, if the source is over 100mbps, it will export it at around 20mpbs.

Does anybody have a clue of what's going on?