NOTE: I set the priority of this issue to Major because I don't know where the RPM install is failing. There could be security implications for a chown command that is not able to execute successfully thus leaving whatever files or directories it was trying to change ownership of with unexpected ownership.
As root, run the command
on the console as it's installing right after the line
So, this is likely either a problem with the MariaDB-server package (since the error message comes right after its "Installing" line) or the MariaDB-backup package (since the error message comes right before its "Installing" line).
I suspect one of these packages incorrectly assumes the mysql user exists. After the install completes, the mysql user exists--I assume because one of the three packages adds it.
If the three packages are uninstalled
and then installed again
then this second time there is no error about the user mysql being invalid. I presume this is because the mysql user is not removed as part of the package removal and therefore the user already exists the second time around.