For some reason, my outlook add-in (uploaded via web interface) is not visible in IOS. I tested the same in Android and it comes up there as expected. I tried on different versions of IOS and all of them behave in the same way. Any help is much appreciated.
I found the answer myself. All you have to do is close the app and reopen, and repeat this a few times until the add-in is loaded. Although this is still a bug with the add-in infrastructure, my workaround helps now.