I’ve been struggling with finding the reason for sessions not saving randomly (and often).
In my case the session is saved on page load:

// in the controller
public function some_view(Request $request)
{
    $request->session()->put('key1', 'value1');

    // ... some more code...

    return view(...);
}

Then on another page or function somewhere when I try to get the key1 value it is null:

// in another method in the project, maybe another controller or mail function
class SomeEmail extends Mailable
{
    public $some_value;

    public function __construct() {
        $this->some_value = session('key1'); // but it's null very often (but not always)
    }
 
}

Then I came across this post of a very old Laravel version (v4): /a/37409073/10847401

which suggests to use the save() method when I put values:

    public function some_view(Request $request)
    {
        $request->session()->put('key1', 'value1');
        $request->session()->save(); // <-- added this
    
        // ... some more code...
    
        return view(...);
    }

The save() method is not in the docs but it still exists in the API for Laravel 10: https://laravel.com/api/10.x/Illuminate/Contracts/Session/Session.html#method_save

And from some reason that worked and there are no longer times where the session value is null in that case

My Laravel project was updated from Version 7 to 10 gradually so maybe that could have also caused some issues?