[once issue]there is large gap between collectionview contents and navigation controller

I have a collection view with custom layout.

and it has different kinds of dynamic contents.

suddenly when i tap on an item, go to next viewController and move back to collection view, there is a large gap between navigation controller and collection view content.

could someone please share how to resolve this issue or how to find out the cause?

please note that, collection view resides in a pageview controller

[once issue]there is large gap between collectionview contents and navigation controller
 
 
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