Concatenating assignment operator loses taint

[Grunny]

Running PHP 7.0.10 using the concatenating assignment operator appears to lose taint.

If I run the following script which concatenates strings normally:

<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');

$a = "tainted string" . ".";
taint($a);
$b = 'some string ' . $a;

echo $b;

I get the following output as expected:

Warning: main() [echo]: Attempt to echo a string that might be tainted in /path/to/taint-test.php on line 9

Call Stack:
    0.0001     356120   1. {main}() /path/to/taint-test.php:0

some string tainted string.

However, if I instead use the concatenating assignment operator in the following script:

<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');

$a = "tainted string" . ".";
taint($a);
$b = 'some string ';
$b .= $a;

echo $b;

The output is simply some string tainted string. and no warning is raised.