Why can the person who was not assigned the task amend it?
On Taiga, task assignment doesn’t necessarily mean that the assigned user owns the task, but rather that they are working on it. The ability to modify tasks depends on your role permissions. Taiga offers a high level of granularity in terms of permissions, allowing you to configure it to suit your needs. You can explore this further in our article on Taiga roles and permissions: Taiga roles and permissions.
I hope this helps! If it is the case, please mark this post as solved.