Mocked class in another test treated as though it exists

Using Laravel 5.8, I have the following 2 tests within the same test class:

/**
 * @test
 */
public function a_procurement_group_can_be_created_or_fetched_from_odoo_for_this_shipment()
{
    $shipment = InboundShipment::make();

    $group = $this->mock(ProcurementGroup::class, function($group) {
        $group->shouldReceive('findByNameOrCreate')
            ->with('foo')
            ->andReturnSelf();
    });

    $this->assertSame($group, $shipment->getProcurementGroup('foo'));
}

/**
 * @test
 */
public function a_procurement_can_be_created_for_an_item_in_the_shipment()
{
    $shipment = InboundShipment::make();

    $group = app(ProcurementGroup::class);

    $procurement = app(ProcurementOrder::class);

    $item = $this->mock(InboundShipmentItem::class);

    $item->shouldReceive('createProcurement')
        ->once()
        ->with($group)
        ->andReturn($procurement);

    $result = $shipment->createProcurementForItem($item, $group);

    $this->assertSame($procurement, $result);
}

The ProcurementGroup class does not actually exist yet, however, in the first test I'm able to mock it and use it. Great.

Now, in the second test, I would expect to receive an error from the $group = app(ProcurementGroup::class); line since the class does not exist, but instead both test pass.

However, if I run only the second test by itself, then I do receive the error that the ProcurementGroup class does not exist.

So it seems that mocking the class in the first test is causing the second test to think the class actually exists. Is this expected behavior?

I was under the impression that since Laravel calls Mockery::close() in the tearDown method of its base test case, that all mock information should be destroyed in between each test.

What is going on here?