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.
Refreshing the browser window and again trying to use the add-in with show the command button label again (but just the first time the add-in is used).
I have been able to reproduce this issue with the Command Demo add-in from the Office Dev site:
1. Clone from GitHub https://github.com/jasonjoh/command-demo
2. Run locally using gulp serve-static (as per instructions in the GitHub repo)
3. Deploy the add-in manifest (as per instructions in the GitHub repo)
On first use of the add-in in a session (or after refreshing the browser window) the add-in title uses the label of the command button. In the case below the button label of “Display all properties” is shown.
If you close the add-in and then click on the same command button subsequent times then the correct add-in title of “Add-in Command Demo” is displayed.
This bug only seems to affect OWA, Desktop Outlook 2016 consistently displays the correct add-in title as shown below.
It would also be nice if the header we displayed in Desktop Outlook was consistent with OWA. As you can see from the screenshots above OWA shows the add-in icon in the header whereas Desktop Outlook just has the title without an icon.
I’ve logged this bug on UserVoice, if it’s causing pain for you please vote it up!