PWA Cookie does not survie restart

My backend uses PHP. I set a cookie with:

if(isset($_COOKIE['syncmarks'])) 
    error_log(8,'Cookie is available');
else
    error_log(8,'Cookie is not set');

$days = 30;
$expire = time()+60*60*24*$days;
$options = array (
    'expires' => $expire,
    'path' => null,
    'domain' => null,
    'secure' => true,
    'httponly' => true,
    'samesite' => 'Strict'
);
setcookie('cookiename', 'somedata', $options);

if(isset($_COOKIE['syncmarks'])) 
    error_log(8,'Cookie is available');
else
    error_log(8,'Cookie is not set');

I "install" the PWA via Firefox on Android, but i see the same behavior with Chrome. If i start the PWA from the Homescreen. After the first start, i see in the PHP Logfile:

Cookie is not set
Cookie is available

After closing the PWA, i start it again. Now i expected something like:

Cookie is available
Cookie is available

But i get again:

Cookie is not set
Cookie is available

With that, i assume, the Cookie has not survived the restart. What do i make wrong?

BTW: Using a standard browser application works as expected. The second output is:

Cookie is available
Cookie is available

source https://stackoverflow.com/questions/77427271/pwa-cookie-does-not-survie-restart