When I create my own MySQL dialect that overrides MySQL56SpatialDialect it ignores my overridden column types. If I change the parent class back to MySQL5InnoDBDialect it works fine.

public class MySQL56InnoDBDialect extends MySQL56SpatialDialect // MySQL5InnoDBDialect
{
    public MySQL56InnoDBDialect()
    {
        super();
        // http://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html
        registerColumnType( Types.TIMESTAMP, "datetime(6)" );
    }
}

With MySQL5InnoDBDialect as the parent I get table columns with datetime(6).
With MySQL56SpatialDialect as the parent I get table columns with datetime.

This maybe related to HHH-10486 Closed