Search Message Extension Works in Teams but not in Outlook

Question

I have a teams application which includes a bot, which works across Teams, Outlook and Microsoft 365. I am adding search messaging extensions to it. The search messaging extension works well within Teams, but in Outlook I receive an error message "Something went wrong".

Looking at the traffic via Fiddler provides an unhelpful error message:

Looking at my webserver traffic I don't believe this request is ever reaching my bot.

I suspect the issue may be to do with the bot channels. This guide implies I need to add the "Microsoft 365 Extensions" channel. However this does not appear as an available channel for my bot via the Azure Bot Services portal:

My bot does not appear in the Bot Framework portal which the guide mentions as an alternative to Azure Bot Services Portal.

Additional Info:

• The rest of the Teams app (Tabs) does appear and work in outlook, only the message extension fails.
• The Teams app manifest.json file is at version 1.16
• The bot "Schema Transformation Version" is at 1.3
• All the suggested client ID's have already been added to the "Expose an API" section of the app registration