That is how almost all Canon raw files look like. It is called optical black/masked pixels. A raw converter crops that field before you see it, but didn’t do it in your case. darktable users can just hit Passthrough in the module Raw black/white point to see the optical black pixels.
For Rawtherapee you find the settings in the file camconst.json
Does your camconst.json file look the same?
Also see this post Measure black and white levels? - #11 by Peter