Didn't receive old passport in post after dropbox

Hi, my old passport had an expired US H-4 Visa stamp, while my new passport did not have any stamp - only arrival/departure records. I submitted these passport booklets and other documents last month during my dropbox appointment.

About a month later, I received the new passport with the stamp in post today, but the package did not have my old expired passport in it.

Has anyone experienced this before? Should I wait for the old passport to arrive in a separate post, or should I contact the USTravelDocs immediately?