I had originally thought this gamma shift problem was for everything imported, including video, but I've been able to boil it down to being just when I import jpg or png files.
I'm thankful that there's at least a workaround in most cases--convert all jpg to bmp or psd files before importing them into Premiere.
This problem started in v13 2019.
Here are some examples. The first image is the original image, with the color values stated on the left side. The second image is what happens when it's rendered/exported through Premiere and I notate the change in color in red on the right side.