After some thinking and a bit of digging I found the following:

When we try to access the attribute, the __get($key) method on our Illuminate\Database\Eloquent\Model is called. This in turn calls the getAttribute($key) method, which does the following:

public function getAttribute($key)
{
    if (array_key_exists($key, $this->attributes) || $this->hasGetMutator($key)) {
        return $this->getAttributeValue($key);
    }

    return $this->getRelationValue($key);
}

So in essence, what it does is it checks if our model either has the attribute set, in which case the model is hydrated, or if we have defined a mutator. Laravel assumes that we do not use mutators on our relation fields. Since I had defined a mutator to map my value to the namespaced class, it attempted to return $this->getRelationValue($key). But since I actually hadn't yet loaded the relationship, and the model hence was not hydrated, I got null.

Having attempted to fix this with

public function getSourceTypeAttribute($type)
{
    // transform to lower case
    $type = $this->getRelationValue('source_type');
    $type = strtolower($type);

    // to make sure this returns value from the array
    return array_get($this->types, $type, $type);

    // which is always safe, because new 'class'
    // will work just the same as new 'Class'
}

I can also state that it does not yet work, but I'm one step closer.

After quite some digging around I have found the issue!

Currently, there is a buildDictionary(Collection $models) in Illuminate\Database\Eloquent\Relations\MorphTo defined as

protected function buildDictionary(Collection $models)
{
    foreach ($models as $model) {
        if ($model->{$this->morphType}) {
            $this->dictionary[$model->{$this->morphType}][$model->{$this->foreignKey}][] = $model;
        }
    }
}

In essence, what this does is take our model(s), and check if they have the attribute of $this->morphType, and $this->morphType is set in the constructor and is by default the caller method's name (in my case source), appended with _type. So, for me, it's source_type. It will then check for $model->source_type, so for me, that'd be $orderItem->source_type. It then takes this and adds to its dictionary, which results in an array like the one beneath, with the following values for demonstration purposes:

$this->morphType = 'source_type'
$model->{$this->morphType} = 'product'
$this->foreign_key = 'source_id'
$model->{$this->foreign_key} = 1 $model = OrderItem::find(1)

[
    'product' => [
        1 => [
            OrderItem {attributes}
        ]
    ]
]

I'll probably fork the laravel core and add the functionality of custom mapping of type and class to resolve my issue.