When a work item is added to an iteration: Question on functionality. Do the Planned dates "stick"

I have used this functionality for some projects added to a Sprint (editing the Agile Pod). I turned in on for a project, completed the project schedule, and then added to a Sprint. I then changed the setting back to " Modify .. Date to match iteration start and end dates." It appears that the planned completion dates for the project did not not stick; I'm thinking that this changed overnight and it looks like the planned dates changed to the Sprint dates. What's the expected functionality?