Hello, you're experiencing the new feature "Share to Teams". It has been delayed several times but now being rolled out. I've read many posts about issues with the new feature and what you're describing is the most frequent one. It seems as if many are getting two add-ins, and only one is working. You actually have both while some only have the one not working, and therefore need to add the other one to the menu bar.
Can't say exactly why this is happening as I haven't heard any official info from Microsoft but surely some form of "bug".