Why are thumbnails bad, when jpgs are good, with raw processed with darktable?

Your image contains an embedded color profile for "PQ Rec2020 RGB". According to the Exif, Darktable appears to be the source of the profile. (See ExifTool output below.)

Some programs ignore embedded color profiles, which results in the washed out appearance of the thumbnails. For better results, select "sRGB (web-safe)" as the output profile for export. Be aware, there are two locations where the output profile may be set. One is set on a per image basis. The other is in the export options. Make sure they are both set correctly.

In the export settings, if you select "image settings" it should use whatever is set for each image. If you put something else, it would over-ride the image settings. Based on information you've provided at the Darktable github site, it appears the images were set for sRGB output, but the export was set for Rec2020, so Rec2020 over-rode the sRGB setting.

If you are meticulous about checking the per image settings, it would be fine to export to "image settings". If you're not sure, it's safer to set it to "sRGB". The only time I would use something else is for 16-bit TIFF output that I'd want to edit in another program. Then when done editing, I would still export to sRGB JPGs.

Another scenario in which it might make sense to export to a different profile is if you are absolutely certain all of your intended output devices are compatible with a different profile, and you either don't care or are certain that everything else is properly color managed. If I were using 100% Apple devices and high-gamut monitors, I would consider using an Apple Display P3 profile, since that is what iPhone already uses by default. But it could cause problems similar to what you've experienced for non-Apple users (eg, images posted to social media).

If changing the output profile does not correct the issue, consider using a different raw-processing program, such as RawTherapee.

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Here is the output from ExifTool regarding the embedded profile of your image.

[ICC_Profile]   Profile CMM Type                : Little CMS
[ICC_Profile]   Profile Version                 : 2.1.0
[ICC_Profile]   Profile Class                   : Display Device Profile
[ICC_Profile]   Color Space Data                : RGB
[ICC_Profile]   Profile Connection Space        : XYZ
[ICC_Profile]   Profile Date Time               : 2020:09:23 19:05:34
[ICC_Profile]   Profile File Signature          : acsp
[ICC_Profile]   Primary Platform                : Apple Computer Inc.
[ICC_Profile]   CMM Flags                       : Embedded, Independent
[ICC_Profile]   Device Manufacturer             : 
[ICC_Profile]   Device Model                    : 
[ICC_Profile]   Device Attributes               : Reflective, Glossy, Positive, Color
[ICC_Profile]   Rendering Intent                : Perceptual
[ICC_Profile]   Connection Space Illuminant     : 0.9642 1 0.82491
[ICC_Profile]   Profile Creator                 : Little CMS
[ICC_Profile]   Profile ID                      : 0
[ICC_Profile]   Profile Description             : PQ Rec2020 RGB
[ICC_Profile]   Profile Copyright               : Public Domain
[ICC_Profile]   Media White Point               : 0.3127 0.32899 1
[ICC_Profile]   Chromatic Adaptation            : 1.04788 0.02292 -0.05022 0.02959 0.99048 -0.01707 -0.00925 0.01508 0.75168
[ICC_Profile]   Red Matrix Column               : 0.67348 0.27904 -0.00194
[ICC_Profile]   Blue Matrix Column              : 0.12505 0.04561 0.79684
[ICC_Profile]   Green Matrix Column             : 0.16566 0.67534 0.02998
[ICC_Profile]   Red Tone Reproduction Curve     : (Binary data 8204 bytes, use -b option to extract)
[ICC_Profile]   Green Tone Reproduction Curve   : (Binary data 8204 bytes, use -b option to extract)
[ICC_Profile]   Blue Tone Reproduction Curve    : (Binary data 8204 bytes, use -b option to extract)
[ICC_Profile]   Chromaticity Channels           : 3
[ICC_Profile]   Chromaticity Colorant           : Unknown (0)
[ICC_Profile]   Chromaticity Channel 1          : 0.70799 0.29201
[ICC_Profile]   Chromaticity Channel 2          : 0.17 0.797
[ICC_Profile]   Chromaticity Channel 3          : 0.131 0.04601
[ICC_Profile]   Media Black Point               : 0 0 0
[ICC_Profile]   Device Model Desc               : PQ Rec2020 RGB
[ICC_Profile]   Device Mfg Desc                 : Darktable