'Permission denied' connecting from instance to database

I cannot connect from my Lightsail instances to the Lightsail database, always 'permission denied'.
I can connect to the same database remotely with public mode enabled.

In my app I have the connection string defined as:

'db' =>
		[
			'dsn'         => 'mysql:dbname=dbmaster;host=endpoint;port=3306',
			'username'    => 'username',
			'password'    => 'password',
			'persistent'  => false,
			'log_queries' => false,
			'reconnect'   => false,
			'queries'     =>
			[
				'SET NAMES UTF8',
			],
		],

Edited by: Lester on Apr 7, 2020 6:05 PM
Preview looks fine, published version not same, special characters converted to html entity codes.