Premiere mangles export of video with several settings
Instead of exporting in Premiere, I chose Queue in the Export dialog for Adobe Media Encoder to do the work. It worked perfectly fine, either with medium or high bitrate, with hardware or software encoding.
This issue may be due to new software running on old hardware. My computer has 8 GB of RAM. Although Premiere 2019-2020 has that amount as the minimum and so it should work, Adobe Support has excused itself of another bug blaming the lack of the recommended RAM.