Currently, if you ‘share’ an item to a collection, even though it may remain in a personal folder, its data and ownership appears to transition to the collection. If someone else with access to the same collection as a given user deletes it, that data is now gone from the vault of the person who created and shared the item to begin with.
Perhaps ‘share’ should be renamed to ‘move’ since sharing implies you’re allowing the collection members to see your item, when in reality it ceases to be a part of your personal vault after sharing.
Regardless of the above, I’d like to see something along the lines of “Share a copy” so you can push an item into a collection while retaining your own copy of that data in your vault for backup purposes. If there’s an identical item in the collection and you share a copy again in the future, it could ask if you’d like to overwrite.