collectionViewのLayoutが崩れる。

前提・実現したいこと

iphone8 でのcollectionViewの表示。

iphone11 pro でのcollectionViewの表示。

iphone8 plus　でcollectionViewCellを3つ並べたい。

発生している問題・エラーメッセージ

iphone11 proではcollectionViewCellが３列並ぶのですが、
iphne8 plusになるとなぜか、layoutが崩れて２列しか並ばない。

デバイスの違いで、描画が変わってしまいます。

該当のソースコード

import UIKit

class CategoryCollectionViewController: UIViewController {

    //MARK: IBoutlet Vars
    @IBOutlet weak var categoryCollectionView: UICollectionView!

    //MARK: Vars
    var categoryArray: [Category] = []

    //MARK: Let
    private let sectionInsets = UIEdgeInsets(top: 20.0, left: 10.0, bottom: 20.0, right: 10.0)
    private let itemsPerRow: CGFloat = 3

    //MARK: View LifeCycle
    override func viewDidLoad() {
        super.viewDidLoad()

        categoryCollectionView.delegate = self
        categoryCollectionView.dataSource = self

    }
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        //TransParency NavigationBar
        self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
        self.navigationController?.navigationBar.shadowImage = UIImage()

    }
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        loadCategories()
    }
    //MARK: Download Categories
    private func loadCategories(){

        downloadCategoriesFromFireStore { (allCategories) in

            self.categoryArray = allCategories
            self.categoryCollectionView.reloadData()
            print("all Category Count : \(self.categoryArray.count)")

        }
    }
}
//MARK: UICollectionView Delegate DataSource
extension CategoryCollectionViewController: UICollectionViewDelegate, UICollectionViewDataSource{

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {

        return categoryArray.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CategoryCollectionViewCell

        cell.toFields(categoryArray[indexPath.row])

        return cell

    }
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {

        performSegue(withIdentifier: "CategoryItemSegue", sender: categoryArray[indexPath.row])

    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

        if segue.identifier == "CategoryItemSegue"{

            let vc = segue.destination as! ItemsTableViewController

            vc.category = sender as? Category
            return

        }
        print("categoryItemSegue can not find")

    }

}
//MARK: CollectionView FlowLayout
extension CategoryCollectionViewController: UICollectionViewDelegateFlowLayout {

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

        let paddingSpace = sectionInsets.left * (itemsPerRow + 1)
        let availableWidth = UIScreen.main.bounds.width - paddingSpace
        let withPerItem = availableWidth / itemsPerRow

        //cellの大きさを返す
        return CGSize(width: withPerItem, height: withPerItem)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {

        return sectionInsets

    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {

        return sectionInsets.left

    }

}

補足情報（FW/ツールのバージョンなど）

ここにより詳細な情報を記載してください。