Laravel: Why does eager loading only keep results for last model?

If I execute the following:

App\Assignment::with(['unit.records' => function ($q) {
                    $q->where('process_date', '>=', '2016-01-01')
                        ->where('process_date', '<=', '2018-09-27')
                        ->orderBy('process_date')
                        ->orderBy('process_hour')
                        ->limit(1);
                    }])
                    ->whereIn('id', [9])
                    ->get();

Then I get the following result:

Illuminate\Database\Eloquent\Collection {#1104
 all: [
   App\Assignment {#1112
     id: 9,
     unit_id: 6,
     start_date: "2015-11-25",
     end_date: "2016-01-04",
     unit: App\Unit {#986
       id: 6,
       records: Illuminate\Database\Eloquent\Collection {#945
         all: [
           App\Record {#853
             id: 6624,
             unit_id: 6,
             process_date: "2017-09-19",
             process_hour: 14,
           },
         ],
       },
     },
   },
 ],
}

Note how the loaded unit has a record that matches the query.

Now, if I use the exact same query but add another assignment (49) to my whereIn array:

App\Assignment::with(['unit.records' => function ($q) {
                    $q->where('process_date', '>=', '2016-01-01')
                        ->where('process_date', '<=', '2018-09-27')
                        ->orderBy('process_date')
                        ->orderBy('process_hour')
                        ->limit(1);
                    }])
                    ->whereIn('id', [9,49])
                    ->get();

The record for assignment 49 is shown, but the record for assignment 9 does not show up anymore:

Illuminate\Database\Eloquent\Collection {#1032
 all: [
   App\Assignment {#1014
     id: 9,
     unit_id: 6,
     start_date: "2015-11-25",
     end_date: "2016-01-04",
     unit: App\Unit {#1283
       id: 6,
       records: Illuminate\Database\Eloquent\Collection {#1254
         all: [],
       },
     },
   },
   App\Assignment {#1061
     id: 49,
     unit_id: 29,
     start_date: "2017-02-24",
     end_date: "9999-12-31",
     unit: App\Unit {#1279
       id: 29,
       records: Illuminate\Database\Eloquent\Collection {#1131
         all: [
           App\Record {#1284
             id: 6062,
             unit_id: 29,
             process_date: "2017-03-10",
             process_hour: 13,
           },
         ],
       },
     },
   },
 ],
}

The record that matches the criteria for assignment 9 obviously exists, but for some reason it doesn't get loaded when the query finds more than one assignment with a unit/record that matches the criteria.

I tested this with more assignments as well, and in each case the record will only show up for the last assignment in the array.

What's the deal here?

via Chebli Mohamed