Share to teams from personal tab not working on mobile

We are developing a thrid-party app for Teams.

We are hoping to use the Share-to-Teams function as described here:

https://learn.microsoft.com/en-us/microsoftteams/platform/concepts/build-and-test/share-to-teams-from-personal-app-or-tab

However this is not working on the mobile client, and we can't get a handle on what is going wrong and the documentation is a bit light.