Laravel 5.1, PHP 7.0: WhereNotIn not working

we have this 2 eloquent script which retrieves a user video based on the filter. I know this is working before because I haven't received any complains from the testers and my superiors have verified it is working.

$previous_winners = Stage::whereNotNull('win_date')
                ->where('win_date','>',Carbon::now()->subDays(8)->format('Y-m-d H:i:s'))
                ->lists('userId')
                ->toArray();

output for the 1st script is like:

array:4 [
  0 => 84
  1 => 361
  2 => 304
  3 => 326
]

but the output of the script below is null

$stage = Stage::whereIn('stageId',$id)
                            ->whereNull('win_date')
                            ->whereNotIn('userId',$previous_winners)
                            ->where('stageId','!=',Config::get('constants.intro'))
                            ->where('stageId','!=',88)
                            ->first();

When the controller is called I always get a null value for the 2nd script even though when I checked the data it should retrieve one as I forced it to return results. The issue that I found out is that when removing this line ->whereNotIn('userId',$previous_winners) i get the correct results but when I put it back again i only get null values. I tried moving the whereNotIn to different positions from the first and before the end but as long as it is present I only get null values. Any idea or workaround on this one, please? Thanks.

via Chebli Mohamed