MySQL (MariaDB) bound to 0.0.0.0, but doesn't appear to be listening to all IPs?
Solved this issue!
It turns out that it was a bug in the local private IP assignment with Ubuntu 18.04 and the host provider, Linode. After looking into things a little deeper, even though the private IP was assigned, manually setting up Netplan instead of the pre-existing automated networking setup resolved the communications issue.