Relationships Not Working Correctly

This is the weirdest thing, maybe someone can help me decipher this.

3 tables for this example but it's not JUST these 3, it's happening in multiple places.

Warehouses, PendingReceipts, ItemUnits

CREATE TABLE `warehouses` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `code` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `name` varchar(40) COLLATE utf8_unicode_ci NOT NULL,
  `status` tinyint(1) NOT NULL DEFAULT '1',
  `created_at` timestamp NULL DEFAULT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `code` (`code`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `pending_receipts` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `order_type_id` int(10) unsigned NOT NULL,
  `complete` tinyint(1) NOT NULL,
  `po` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
  `line` int(10) unsigned NOT NULL,
  `qty_ordered` int(10) unsigned NOT NULL,
  `qty_received` int(10) unsigned NOT NULL,
  `item_code` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `item_description` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
  `ship_to` varchar(10) COLLATE utf8_unicode_ci NOT NULL,
  `project_number` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,
  `note_title` varchar(80) COLLATE utf8_unicode_ci DEFAULT NULL,
  `note` text COLLATE utf8_unicode_ci,
  `process_text` varchar(70) COLLATE utf8_unicode_ci DEFAULT NULL,
  `created_at` timestamp NULL DEFAULT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `pending_receipts_order_type_id_foreign` (`order_type_id`),
  CONSTRAINT `pending_receipts_order_type_id_foreign` FOREIGN KEY (`order_type_id`) REFERENCES `order_types` (`id`) ON DELETE NO ACTION ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=331 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `item_units` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `pending_receipt_id` int(10) unsigned NOT NULL,
  `rfid` varchar(16) COLLATE utf8_unicode_ci NOT NULL,
  `batch` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `item_code` varchar(20) COLLATE utf8_unicode_ci NOT NULL,
  `item_description` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
  `unit_type` varchar(1) COLLATE utf8_unicode_ci NOT NULL,
  `qty` int(10) unsigned NOT NULL,
  `warehouse` varchar(10) COLLATE utf8_unicode_ci NOT NULL,
  `status` int(10) unsigned NOT NULL,
  `rack_location` varchar(7) COLLATE utf8_unicode_ci DEFAULT NULL,
  `project_number` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,
  `po` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
  `line` int(10) unsigned NOT NULL,
  `order_type` int(10) unsigned NOT NULL,
  `callout_id` varchar(16) COLLATE utf8_unicode_ci DEFAULT NULL,
  `active` tinyint(1) NOT NULL DEFAULT '1',
  `created_at` timestamp NULL DEFAULT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `rfid` (`rfid`),
  KEY `item_units_pending_receipt_id_foreign` (`pending_receipt_id`),
  KEY `item_units_batch_index` (`batch`),
  KEY `warehouse` (`warehouse`),
  CONSTRAINT `item_units_pending_receipt_id_foreign` FOREIGN KEY (`pending_receipt_id`) REFERENCES `pending_receipts` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1254 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

The first thing to notice is that is no foreign key or index on pending_receipts.ship_to while there are keys on item_units.warehouse and warehouses.code

The pending_receipts.ship_to is a warehouse code that may or may not match one of the warehouses.code rows, wherein, the warehouses table is a listing of our warehouses, but not all ship_to warehouses are ours, so there is no foreign key to refrence these.

Yet in my PendingReceipt model, I have a belongTo relationship to the warehouses table:

public function warehouse() {
    return $this->belongsTo(Warehouse::class, 'ship_to', 'code');
}

Inversely on my Warehouse model I have a hasMany relationship:

public function receipts() {
    return $this->hasMany(PendingReceipt::class, 'ship_to', 'code');
}

When I eager load either model, both return perfectly with the properly loaded relationships.

$warehouses = Warehouse::with('receipts')->get(); // loads all pending_receipts associated with the warehouse

$receipts = PendingReceipt::with('warehouse')->get(); // loads the warehouse associated with the receipt

This seemed strange since there is no foreign key or index on the ship_to column. So my though was ok, perhaps you can just use the column names without keys. But alas, even if I add a key to my item_units table ie the warehouse column I cannot get these relationships to function.

There should be a relationship between the warehouses.code column and the item_units.warehouse column. In my Warehouse model:

public function items() {
    return $this->hasMany(ItemUnit::class, 'warehouse', 'code');
}

// and in the ItemUnit model

public function warehouse() {
    return $this->belongsTo(Warehouse::class, 'warehouse', 'code');
}

Eager loading either of these 2 models I get no relationships created at all, despite the fact that it works perfectly on the other two.

Both:

$items = ItemUnit::with('warehouse')->get(); // warehouse relation is null

$warehouses = Warehouse::with('items')->get(); // items relation is null

What the heck am I missing, why does one of these work perfectly and the other not work at all?

Thanks.