/system/src/Grav/Common/Page/Collection.php::adjacentSibling() may return back the whole Collection if previous or next sibling is not found. All other places in such case expect to return false. system/src/Grav/Common/Page/Interfaces/PageCollectionInterface.php::adjacentSibling() defines return types PageInterface|PageCollectionInterface|false, but PageCollectionInterface should not be there as no other method expects to return it. Also naming suggests a PageInterface if found or empty value (null|false) to be returned if not found, but not a collection