When a record is submitted for approval. Let's say this approval request is assigned to 3 users. The 'Actual Approver' column in Approval History is populated with the 3 assigned user's name before actual approval action is made. Is this the salesforce expected behavior? But once the actual approval (Approve / Reject) is done by any user, the 'Actual Approver' column in Approval History is populated only with the user name who took the action. This is working as expected. Please refer images below
This is expected behavior. Here in a single step you have assigned to user and if one of the user is approving the request the step is showing the status as approved.
If I understand correctly, you need all the assigned approvers should approve the request and then the final status should be Approved. In that case, you need to define separate approval processes.
There, after first user approves the request, you will automatically submit the request for 2nd approval through process builder.