teams app static tab not sending tab/fetch until opened in pop out app

Question

I have a teams bot with a static tab. I have a number of customers using them and the static tab loads corectly for all of them except one. With one customer however, when they open the tab nothig happens and the tab contents remain blank. If they then press on the ":pop out app" button the pop up opens and does successfully populate the tab.

It seems as if the bot never receives the tab/fetch message when the static tab is opened.

This happens consistently for all users within this customer, (and this customer only), if they are connected to their firewall or not. It happens in both the app and the browser based version of teams.

Has anyone encountered such an issue?