Invoice status does not change to "viewed"

I’ve read the other thread here about invoice status not changing to “viewed” after viewing with the guest URL (that was due to the admin using the same browser to view the invoice - expected behavior).

In my case, I contacted a customer and had him view his invoice online from his location. Still the invoice show status as "sent’ - it does not change to “viewed”. Any ideas what may be the cause of this?