[Webhook] Why does the assignee changes everytime in MR after a push?

Hello ! I am trying to push some notification on our team communication tool everytime the assignee is changed, using webhook on Merge Request events.
To do that, I am checking whether the object changes contains the object assignees, with the property current and previous for the old and new assignee.
The problem is that each time someone is pushing code to the branch in MR, the object changes contains an assignee with the property previous empty and the current to the actual assignee, so we have notification everytime…

Is this the expected behavior ? What would be the best way to implement such use case ?