Microsoft Teams Bot not receiving messages

We have created a Bot through the Teams Dev Portal (https://dev.teams.microsoft.com/) and attached it to a Teams Tab app, but the bot is not receiving any messages from the Bot Framework. The Tab app was also created using the Teams Dev Portal and published to the Org app store.

The same bot works fine if we test it on Bot Framework Portal (https://dev.botframework.com/).

The bot is a http service hosted on AWS and registered in Azure AD.

We have a couple of other virtually identical bots created a few months ago that continue to work fine. Newly created bots seem to have this issue.