I just figured out that when I've got my external GPU connected, any encoding to h264 or h265 using Apple's Compressor gets softened and slightly offset down and to the right. It's subtle so I didn't notice for a month or so.
Can anyone else reproduce this? I'm using a Radeon RX5700XT in a Razer Core enclosure, source files are ProRes. Doesn't happen encoding h264 directly from FCPX, or in Handbrake, or to ProRes in Compressor. I've already got the "Use GPU to process FCP content sent to Compressor" checkbox turned off. Happens whatever the encode settings in Compressor are, with no resizing, changes to frame rate, etc, just as long as it's going to h264/h265.