It seems there is a bug with the Outlook add-in commands when using the add-in from Outlook Web Access.

When using the add-in command (ribbon button) to open a task pane to show your add-in the label of the command button is shown at the top of the add-in where the add-in title should be. If you close the add-in and use the command button subsequent times in a session, then the add-ins title is correctly displayed.

Full steps to reproduce the issue can be found on my blog:

https://camerondwyer.wordpress.com/2016/12/19/bug-in-outlook-add-in-commands-showing-command-label-instead-of-the-add-in-title-on-first-use-in-a-session