Nope, I created them and set the password drop down to "Set to..." then set the password. I started the VM back up today to check on labels for this post and it now works without a problem. I'm not sure what happened but I cannot replicate it now. Perhaps I accidentally set the user to unix validation without realizing it like you suggested.
non-root webmin users unable to log into Webmin
Essentially the same issue. The dashboard is also broken for me, simpy displaying a message like system-status::list_system_info failed : Hour '51' out of range 0..23 at /usr/libexec/webmin/proc/linux-lib.pl line 59 OS: CentOS 8.0.1905 Webmin: 1.930