Tasks in Fieldlens are designed so that an assignee can never close themselves out on the work assigned. Instead, when an assignee has completed their task, they mark themselves Resolved.
To that end, assignees will have a slightly different view of a post than the person that assigned the task to them - namely, there will not be an option to Close themselves. All details, photos, and information will be the same and the assignee can add comments, more media, etc. But they will only be able to mark themselves Resolved.
Note the two views of the same post within Fieldlens. The assignee can not close the task nor edit their due date - both options are greyed out.
Once the assignee marks themselves resolved, the person that assigned the task will be notified and then be able to either Close the assignee, or reopen them if more work is needed.