This seems to be something that has recently changed for all of the TR and SR tenants that I have tested this on. As far as I'm aware SharePoint should be updating the Editor comment when an approver approves a file.
We now have several approved files where the approver is not the 'Modified By' of the document. List items appear to work fine still. This could potentially cause chaos for any systems dependent on that logic being in place in SPO.
Approvals also appear to intermittently not be writing into the version history for documents also.
I have attached a video demonstrating the issue as I see it.