Move the crop points slightly so that the squares are better centered in the patches before creating the profile. The problem seems to be associated with the X-Rite color checker autodetection. Even though the squares visually appear to be located within the color patches, part of the border is likely being used to create the profile.

RawTherapee developers provide guidance on shooting color calibration targets to create camera profiles. Notably, do not "fill the frame as much as possible", as some here have suggested. Rather, it's better to:

Position the target so that it fills the center-third of your frame - not more, not less. The center of the frame has the best optics and lowest vignetting.

The DCamProf developer concurs:

Uneven lighting is a common problem in camera profiling. The typical recommendation is to make sure you have even lighting (at least two lights if not shooting outdoors) and shoot the target small in the center (to minimize vignetting).

You can also try other software:

• Adobe DNG Profile Editor
• DCamProf – open source, command line utility.

Here's the output from RawTherapee with the standard DCP tone curve and the following settings:

1. No profile.
2. Camera Standard.
3. DCP file after adjusting crop points.
4. DCP file without moving crop points (autodetect).