Unable to Create a Custom Tab in Excel Web Add-in Using <CustomTab> Element

Hello,

I am developing an Office Web Add-in for Excel and encountering difficulties with creating a custom tab in the ribbon using the <CustomTab> element in the manifest. The tab appears as expected when using the built-in Office tabs (like TabHome, TabInsert, or TabAddIns), but does not show up when attempting to create a custom tab.

Issue Details

Manifest Setup:
I’ve followed the Microsoft documentation and used the <CustomTab> element within the <PrimaryCommandSurface> to define a new custom tab. 

When I use an OfficeTab element instead of CustomTab, assigning it to TabAddIns, TabHome, etc., the add-in loads without any issues, and the button appears as expected. However, switching to <CustomTab> with a unique ID like MyCustomTab causes the tab to disappear from the ribbon in Excel Web (works fine in Excel Desktop).

Troubleshooting Steps Taken

1. Validation: Checked the manifest file for errors. All schema validation passes except when switching to CustomTab, where the tab does not appear.
2. Cache Clearing: Cleared Office cache and reloaded Excel Web, but the issue persists.
3. Documentation & Community: Referenced Microsoft’s documentation, forums, and the Script Lab example. Script Lab and tutorials do not address this specific issue with Web Add-ins.
4. Other Attempts: I tried different ID values, double-checked resource strings, and verified that all file paths and URLs are accessible. The issue only occurs with a CustomTab in the Excel Web environment.

Additional Observations

• The <CustomTab> element documentation suggests that it should be supported across all platforms, but my experience is inconsistent in the Web environment.
• Older videos and tutorials imply that <CustomTab> should work, yet these examples appear limited to desktop environments.

Request

Could someone confirm if there are known limitations with <CustomTab> in Excel Web Add-ins? Additionally, if there is any workaround or specific manifest structure needed to enable this functionality on Excel Web, I would appreciate any guidance.

Additional Context: I am developing in JavaScript with a setup targeting both desktop and web environments. If necessary, I can provide additional manifest details or screenshots.

Thank you for your assistance!