A task is assigned to multiple users, but not all users are required to action the task. When the required number of users open the task, the users that opened the task have their task status changed to 'Open', and the other users should have their task's status changed to 'Allocated.'
However, after upgrading K2 from 4.7 to 5.2, all the task statuses changed to 'Open' whenever one user opened the task.
This issue is already fixed in K2 5.3. If there is this issue on 5.2, you can log a ticket and reference this KB to request the proposed fix for this issue.