laravel 5.8.38 weird route issue with wildcard

I'm running a laravel 5.8.38 on nginx and have this weird route issue

the route is very simple

Route::get('/{appKey?}', 'AppSetupController@index');

that's it nothing more

the {appKey} is something created from time to time by our users with this php function

uniqid("fsapp");

so the appkey will be 18 digits characters like this, and the full url is something like this

mydomain.com/fsapp6268ad9d42fc3

we have thousands of this and everything work well except this one, this one just go straight to 404 i tried to trace the issue on the file AppSetupController on function index, but this exact appKey doesn't even land on this function, it goes straight to 404 and i don't even know how to trace this issue

if i just change even 1 character from this fsapp6268ad9d42fc3 it will go to the correct route and it just work as intended only this one i don't know why always go to 404

via Chebli Mohamed