Custom Teams app works ONLY with Android Teams app

Hello

I have created a simple single-web-page-custom-app for Teams. The custom app works inside Android Teams app but NOT on desk top or any browser.

I'm not sure if target web page allows embedding in iframe. Is this even relevant though, if the app works fine with any Android from phone to tablet?

Everyone else reporting these problems seems to have the opposite: app works on desktop, but not on mobile..