It seems that on EL8 the MariaDB rpm is not providing mariadb-server (mind the case) as it does on EL7:
Checking the RPMbuild code:
Shows indeed that 8 is not listed.
We are running against problems with Plesk for example. But I think there might be a lot more 3rd party rpms relying on mariadb-server.
I don't know if that has todo anything with dnf module fiasco.