Blank Custom Collection Cell in Programmatic UICollectionViewController

I am trying to implement a UICollectionViewController programatically (NOT using the storyboard, although my project does use a storyboard for other things). I am able to connect my data source and display the number of cells that correspond to the number of hits from my index, but the cells are blank and generic; my custom cell doesn't appear as expected.

import UIKit
import InstantSearch

private let reuseIdentifier = "CollectionGridCell"

class ResultsCollectionViewController: UICollectionViewController, HitsController {

    var hitsSource: HitsInteractor<Test>?
    
    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView.register(CollectionGridCell.self, forCellWithReuseIdentifier: reuseIdentifier)
        collectionView.backgroundColor = .clear
        // Do any additional setup after loading the view.
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
 
        return hitsSource?.numberOfHits() ?? 0    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! CollectionGridCell

        // Configure the cell
         let hit = hitsSource?.hit(atIndex: indexPath.row)
        cell.backgroundColor = UIColor.red

        return cell
    }
}

This UICollectionTableViewController is part of a StackView. From the parent VC, it's initialized like this:

 let hitsCollectionController = ResultsCollectionViewController(collectionViewLayout: UICollectionViewFlowLayout())
 stackView.addArrangedSubview(hitsCollectionController.collectionView)

This is what I see. Note that the number of squares accurately reflects how many items I have in my data source.

Here is my xib file, if that helps any.

What I expected to happen is that the cells should have a label with "Test" inside them that I can set based on the hit. Instead, they're just blank and seem like the default cells. My cell isn't registering/displaying properly. I have a feeling it has something to do with the layout settings, but I'm really not sure. I have a corresponding table view that works just fine.

You've created the cell in a nib file so you need to register it with the nib initializer by providing the nib file name.

collectionView.register(UINib(nibName: <#T##String#>, bundle: nil), forCellWithReuseIdentifier: reuseIdentifier) // provide the nib file name in the placeholder