Looking at the business rule it should do it:

if (current.state.changes() && current.state=='approved') {
   updateTask(current, current.approver.getDisplayValue() + " approved the task.", current.comments);
}

And the function "updateTask" is basically getting the task for this approval request and adds the text to the approval_history field.

As far as I can tell the Business Rule is unchanged and out-of-box.

I double checked his with a demo instance and we have the same script.

However, the script also checks a system property — maybe that is set to "false" in your case?

glide.workflow.user_approval_history

https://demo022.service-now.com/nav_to.do?uri=sys_script.do?sys_id=aea34678c61122710151b1b7fdf65762

Also I tried it and even though the comment is added to the field "approval_history" it shows up as a "Additional Comments" entry … not sure why but didn't do any deep digging yet either