Related model returns null in Eloquent, when there are multiple instances of main model

I have a situation where belongsTo realationship is null in the Eloquent model.

Here is my query:

$lead = Lead::where([
  'email' => $input_data['email']
])->firstOrFail();

Also here is the class definition for Lead. It is related to another model.

class Lead extends Model {
  protected $table = 'leads';

  public function broker() {
    return $this->belongsTo('App\Broker');
  }
}

class Broker extends Model{
  protected $table = 'brokers';

  public function leads(){
    return $this->hasMany('App\Lead');
  }
}

I was under the impression that the email is a unique field in the database, but it was not, so there are situations where the query I am running has multiple results. But there is a firstOrFail(), so it should not matter.

The problem is that the broker relationship is null when there are multiple results for the query

$lead->broker //holds a broker object, if there was one result in the query

$lead->broker //is null when the query returned more than on row

Very strange. Am I doing something wrong, or is it a bug?

The project is built with Laraval 5.2.45

via Martin Taleski