samething happened to me, as I check back on the first email I got from USCIS, the part where the tracking number was has become:

(The USPS tracking number assigned is #DCN. You can use this number at <a href="https://tools.usps.com/go/TrackConfirmAction_input?origTrackNum=#DCN" target="_blank">www.usps.com</a>)

But I was able to find the clicked page on my history and find the usps link that showed the tracking information.

To my knowledge, they send out approval letters on regular mail (usually 2-3 days) but they send out cards on express/expedited mail with signature confirmation, so you might receive the card and approval letter at the same time.