I have no idea. My OS is Ubuntu 19.10. I will invesitgate it tomorrow. Unfortunately I don’t have access to my PC at the moment. Meanwhile I think you can load the jpg file as a sidecar from the lighttable.
Edit: It seems that darktable doesn’t save the changes in sidecar until you either switch to another photo or quit the software. I uploaded the sidecar again and hopefully it is OK now.