Finish Behavior
When you mark an open submission as Done/Closed/Denied, the submission gets deleted from the submission channel and will be recreated in the closed or denied (see submission flow).
This has the advantage that your submission channel contains only open submissions. But it also has the disadvantage that attached threads will be lost. To work around that issue, Submitty as the ability to not delete the submission on finish. Submitty will only disable all buttons to prevent further interactions with the submission.
This behaviour can either be changed via the homepage or via the command /channel submission edit