QHelpEngineCore::copyCollectionFile doesn't copy all content of help collection file

We use `QHelpEngineCore::copyCollectionFile` to copy a help collection to a temporary directory and then open and change it in that location to avoid modifying the original collection.

This works fine in Qt 5.15, but now no longer works after migrating to Qt 6.5.5. Attached is a minimal example (based on the "contextsensitivehelp" example) that reproduces the issue. The strange thing is that the search mechanism still works, but the index model/widget and the actual content is empty (see attached screenshots).