When extracting a colorful frame in kdenlive, then adding the resulting png to the timeline, a clearly noticeable change in colors is visible, both in the preview and in the rendered video. The extracted colors are slightly darker and gradients aren't as smooth. Perhaps the color depth is lower?
Tested with Kdenlive 18.08.3. Extracted png has a bit-depth of 32. I see no difference with the clip. It seems the problem is solved. I closed this bug. If it still appears in the latest version, please feel free to re-open it and update the affected version number.