Update August 19, 2019 - Solution/Workaround

Since logging this issue I found a solution/workaround by using the systemLayoutSizeFitting(_:withHorizontalFittingPriority:verticalFittingPriority:) method. It still looks that there is an issue in iOS 13 with systemLayoutSizeFitting of a UICollectionViewCell because this method is no longer being called as it is on iOS 13 and lower. Now you have to override the systemLayoutSizeFitting(_:withHorizontalFittingPriority:verticalFittingPriority:) method of an UICollectionViewCell and use it as you used to use the systemLayoutSizeFitting.

Code example:

import UIKit

class FlickPhotoCell: UICollectionViewCell {

    func getCellSize(_ targetSize: CGSize) -> CGSize {
        return CGSize(width: 50, height: 200)
    }
    
    // Only this is called on iOS 12 and lower
    override func systemLayoutSizeFitting(_ targetSize: CGSize) -> CGSize {
        return self.getCellSize(targetSize)
    }
    
    // Only this is called on iOS 13 beta
    override func systemLayoutSizeFitting(_ targetSize: CGSize, withHorizontalFittingPriority horizontalFittingPriority: UILayoutPriority, verticalFittingPriority: UILayoutPriority) -> CGSize {
        return self.getCellSize(targetSize)
    }
}

systemLayoutSizeFitting issue in iOS 13 beta

This project illustrates an difference in the way the systemLayoutSizeFitting of a UICollectionViewCell is being called when running the same code on iOS 12.1 and iOS 13.0 beta (Xcode version Version 11.0 beta 2 (11M337n)). This leads to the inability to implement a "self-sizing: cells inside a UICollectionView when running on iOS 13. Also made a thread in Stackoverflow regarding this issue.

The systemLayoutSizeFitting is used to implement the so called "self-sizing" cells inside UICollectionView:

iOS 12.1 (the size of the cell is taken from systemLayoutSizeFittingSize:)

iOS 13 beta (the size of the cell is taken from collectionView:layout:sizeForItemAtIndexPath:)