Unreliable Query results from an Access DB with a PHP PDO odbc connection using the MDBTools driver

Question

I'm on a Ubuntu 16.04 box, and have connected to an MS Access database via PHP PDO. I am using the MDBTools odbc driver. To prepare for this, I ran the following commands in the terminal:

sudo apt install php7.0-odbc
sudo apt install libmdbodbc1
sudo service apache2 restart
sudo ln -s /usr/lib/x86_64-linux-gnu/libodbccr.so.2 /etc/libodbccr.so

I can connect to the database just fine. Connection example:

$driver = 'MDBTools';
$accdb  = '/var/www/html/access/mydb.accdb';
$dbh = new PDO('odbc:DRIVER=' . $driver . '; DBQ=' . $accdb);

First query:

// See a whole record
foreach( $dbh->query('
    SELECT 
        * 
    FROM Assets
    WHERE Asset_ID = 12
', PDO::FETCH_ASSOC) as $row )
{
    echo '<pre>';
    var_dump( $row );
    echo '</pre>';
}

Results of first query:

array(7) {
  ["Asset_ID"]=>
  string(2) "12"
  ["Asset_Date"]=>
  string(17) "10/07/16 00:00:00"
  ["Asset_Category"]=>
  NULL
  ["Asset_SubCategory1"]=>
  NULL
  ["Asset_Spot_X"]=>
  NULL
  ["Asset_Spot_Y"]=>
  NULL
  ["Asset_Spot_Z"]=>
  NULL
}

Notice that in the first query, many of the fields are showing that the values are NULL, but this isn't actually the way it is in the database. I'll show you in the second query:

// See same record's Asset_Spot_X value
foreach( $dbh->query('
    SELECT 
        Asset_Spot_X 
    FROM Assets
    WHERE Asset_ID = 12
', PDO::FETCH_ASSOC) as $row )
{
    echo '<pre>';
    var_dump( $row );
    echo '</pre>';
}

Results for second query:

array(1) {
  ["Asset_Spot_X"]=>
  string(4) "1252"
}

These queries are run at the same time, so the value for Asset_Spot_X should be the same, but in one I get NULL, and in the other I get 1252. What am I doing wrong? It seems like the queries that I'm doing are really simple, so I don't understand what's going on. PDO isn't showing any errors when I check for errorInfo and errorCode.