iOS, Swift4: scrollToItem & selectItem not centering the collectionView vertically, but at the bottom

Question

I try to center with animation a cell in collectionview vertically, but it doesn't work. I am using this code:

self.slotCollectionView.selectItem(at: indexPath, animated: true, scrollPosition:  UICollectionViewScrollPosition.centeredVertically)
self.slotCollectionView.scrollToItem(at: indexPath, at: .centeredVertically, animated: true)

Every time I use this, it centers at the bottom of the collection view. I am sure that the selectItem selects the correct cell. It is just a problem of bad positioning…

What can I do?