MySQL (MariaDB) bound to 0.0.0.0, but doesn't appear to be listening to all IPs? My MySQL (actually MariaDB) server accepts connections at port 43210 without problems on the public IP address of, we'll say, 123.123.123.123 on eth0. This is on an Ubuntu 18.04 server that also has 192.168.1.10 configured for eth0.
I have a user configured to with permissions for all hosts (%):
+------------------+-----------+
| user             | host      |
+------------------+-----------+
| myuser           | %         |
...

From a remote server (another host that is on the same LAN) I can connect to the public IP address of this MariaDB server:
$ mysql --user=myuser -pMYPASSWORD --host=123.123.123.123 --port=43210 mydatabase
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 340
Server version: 10.2.31-MariaDB-1:10.2.31+maria~bionic-log mariadb.org binary distribution
...

However, the problem is that when I try to connect to the LAN IP address, mysql times out:
$ mysql --user=myuser -pMYPASSWORD --host=192.168.1.10 --port=43210 mydatabase
ERROR 2003 (HY000): Can't connect to MySQL server on '192.168.1.10' (110 &quot;Connection timed out&quot;)

Just to verify, netstat -tlpn | grep mysql shows it is listening on 0.0.0.0 which should be listening on all interfaces (I believe including the local IP):
tcp        0      0 0.0.0.0:43210           0.0.0.0:*               LISTEN      964/mysqld

I also run PostgreSQL on this same server with MariaDB and if I connect remotely from the same other test host from which I'm trying to make the mysql connection, it connects without a problem:
$ pg_isready -d mydb -h 192.168.1.10 -p 43211 -U myuser
192.168.1.10:43211 - accepting connections

The point of the last note about PGSQL is that I just want to clarify that connections are able to get through to the actual Linux server itself, just not MySQL (MariaDB).
I have also disabled the UFW firewall temporarily in order to keep that out of the equation.
What could be going wrong here?  What can I test further to isolate the problem?
I suspect that perhaps the bind-address = 0.0.0.0 in /etc/mysql/mariadb.conf.d/50-server.cnf may be the problem, but I thought that the quadruple 0 was in fact able to handle this.
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.