Up to Flarum beta 13, Flarum was not setting the SameSite attribute on cookies. This means different browers might have read them as either None or Lax.
Since beta 14, Flarum sets SameSite=Lax by default.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie/SameSite
You can change the SameSite value by editing config.php, for example:
<?php return array (
'debug' => false,
'database' =>
array (
/* existing stuff */
),
'url' => 'https://example.com',
'paths' =>
array (
/* existing stuff */
),
'cookie' => [
'samesite' => 'none', // `strict` / `none`, defaults to `lax`
],
);
