Also, when you say "HTTPS servers set DF," are you observing from the server or from the client? Over the Internet, it's likely that you'd see different values

Also, when you say "HTTPS servers set DF," are you observing from the server or from the client? Over the Internet, it's likely that you'd see different values for DF on opposite sides of the connection. Any router can set or clear the DF bit on your packets for any reason.