Description of problem:

on a full updated centos stream 8 system:

dnf copr enable {{ your favorite }}

does not enable the stream 8 repo, instead
the epel8 one is configured in /etc/yum.repos.d

means - the baseurl in the repo file has 

baseurl=https://download.copr.fedorainfracloud.org/results/.../epel-8-$basearch/

instead

baseurl=https://download.copr.fedorainfracloud.org/results/.../centos-stream-8-$basearch/



How reproducible:

Just pick a favorite copr project that builds for epel8 and stream8 and 

dnf copr enable {{ your favorite }}