Scroll two collection view at one time, bug IOS 15

Does it occur when you scroll collection 1 and when you scroll collection 2 at first time ? What are the constrains for each collectionView ?

I created a simple VC with 2 collectionViews, no constraint defined:

Code of the VC is very basic:

import UIKit

class TestViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {
    
    @IBOutlet weak var collection1: UICollectionView! // delegate and dataSOurce defined in IB
    
    @IBOutlet weak var collection2: UICollectionView!
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        20
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let id = collectionView.tag == 1 ? "Standard1" : "Standard2"
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: id, for: indexPath)

        for v in cell.contentView.subviews {
            v.removeFromSuperview()
        }

        cell.backgroundColor = collectionView.tag == 1 ? .red : .systemYellow
        
        let label = UITextView(frame: CGRect(x: 5, y: 4, width: 30, height: 20))
        label.text = String(indexPath .row)
        cell.contentView.addSubview(label)
        
        return cell
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()

        collection1.tag = 1
        collection2.tag = 2

    }
    
}

I run in iOS 15.0 simulator with Xcode 13RC and at start I can scroll first collection without moving second.

So, you have probably defined something else in your project that causes this behavior.

Re reading your post :

under first and should be partially hidden under screen

What do you mean by "under screen" ? Do you mean below screen limit ? under first Collection ?

I tested both cases, without replicating the problem.

To fully answer the question, I added a tableView with 2 rows, each containing a CollectionView.

It still works OK, even at first scroll, as the screen capture illustrates:

Code for creating the collections:

import UIKit

class TestViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UITableViewDelegate, UITableViewDataSource {
    
    @IBOutlet weak var collection1: UICollectionView!
    
    @IBOutlet weak var collection2: UICollectionView!
    
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        2
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        
        let cell = tableView.dequeueReusableCell(withIdentifier: "CellItemForCollection", for: indexPath)
        
        let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
        layout.sectionInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
        layout.itemSize = CGSize(width: 50, height: 50)
        
        let collectionView = UICollectionView(frame: CGRect(x: 0, y: 0, width: 260, height: 120), collectionViewLayout: layout)

        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "MyCell")
        collectionView.backgroundColor = UIColor.clear

        collectionView.dataSource = self
        collectionView.delegate = self
        
        collectionView.tag = 10 + indexPath.row // 10 or 11
        cell.contentView.addSubview(collectionView)

        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        20
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        var id = ""
        switch collectionView.tag {
        case 1 : id = "Standard1"
        case 2 : id = "Standard2"
        case 10, 11: id = "MyCell"
        default: break
        }
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: id, for: indexPath)

        for v in cell.contentView.subviews {
            v.removeFromSuperview()
        }

        switch collectionView.tag {
        case 1 : cell.backgroundColor = .red
        case 2 : cell.backgroundColor = .systemYellow
        case 10: cell.backgroundColor = .systemBlue
        case 11: cell.backgroundColor = .systemGreen
        default: break
        }
        
        let label = UITextView(frame: CGRect(x: 5, y: 4, width: 30, height: 20))
        label.text = String(indexPath .row)
        cell.contentView.addSubview(label)
        
        return cell
    }
    

    override func viewDidLoad() {
        super.viewDidLoad()
        collection1.tag = 1    // The top collections (out of TableView)
        collection2.tag = 2
    }
    
}

Did you solve the issue ? If not, could you explain what is different from the set up I tested ?