phpmyadmin + redis session save compatibility?

I'm facing a strange behavior on phpmyadmin + redis php session cache. Here is my php-fpm pool configuration :

php_value[session.save_handler] = redis
php_value[session.save_path]    = "tcp://127.0.0.1:6379"

This configurations works fine for a simple php script :

<?php
//simple counter to test sessions. should increment on each page reload.
session_start();


$count = isset($_SESSION['count']) ? $_SESSION['count'] : 1;

echo $count;

echo "<br>";

$_SESSION['count'] = ++$count;


$username = $_SESSION['username'];
if(isset($_SESSION['logged']) && $_SESSION['logged']=='yes') {
    echo "$username";
}   

$_SESSION['username']='Teddy';
$_SESSION['logged']='yes';



?>

I see the increment and the Teddy message.

But when I try to access to phpmyadmin through 'http://myserver/phpmyadmin' I always get the authentication page, even after successfull login, I'm always redirected to that login page...

If I switch to path sessions :

php_value[session.save_handler] = files
php_value[session.save_path]    = /var/www/vhost/sessions

Everything works well (my php script and phpmyadmin).

I would like to know if somebody experienced a similar issue and if there is a fix ? Could this be a limitation of phpmyadmin ?

I'm runing on Linux Debian 8.6 + phpmyadmin 4.2.12-2 + php5-fpm 5.6.24 + redis-server 3.2.4-1.

My vhost is configured this way :

Alias /webops /usr/share/phpmyadmin
...


<FilesMatch "(^$|\.php$)">
        SetHandler "proxy:unix:///var/run/php-fpm/vhost.sock|fcgi://vhost/"
</FilesMatch>

<Proxy fcgi://vhost/$1 >
</Proxy>

RewriteCond %{REQUEST_FILENAME} .php$
RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_URI} !-f
RewriteRule (.*) - [H=text/html]
SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1

Thank you Bests Julien