I am missing the drop in feature and my Alexa contacts after upgrading my iPhone (7 plus to 8 plus). Restarting the app does not help. I know I am logged in and connected because I can see the history of commands on all of my devices and see that they are connected.
I just spoke with Amazon support and came up with a solution. I had to logout of the Alexa app by going to the bottom of the settings section and signing out. I then logged back in, selected the conversation bubble at the bottom of the screen (drop-in feature was still missing), selected the profile icon (person at top right), and finally selecting the conversation bubble at the bottom again. That caused it to reappear along with all of my previous contacts.
Hope this helps someone.