How Assigned to user was able to change the assigned to field, As of now it is read only

[ashutoshwor]

Hello,

For a GBS Case, the assigned to user was able to change the assigned to field.
As of now the field is read only now the users are not able to do so.

So what could be the reason the users were able to change.
And kindly refer the attached screenshot what is that human like icon next to the assigned to field.