From link you shared from previous discussion site:
0.096445 [opencl_init] discarding device 0 `gfx902:xnack+’ due to insufficient global memory (512MB).
If you’re installing from Fedora Repos, you can change the Darktable memory requirement from file ~/.config/darktable/darktablerc.
Since you’re using flatpak, you can find the config inside /home/<yourusername>/.var/app/org.darktable.Darktable/config/darktable/ and find darktablerc.
Find opencl_memory_requirement=768 and change to 500. And test again. Usually with rpm package we can test with /usr/bin/darktable-cltest but I don’t know how with flatpak. doc source.
Edit: change the doc resource to be more relevant with current Darktable 3.8 version.