How to disable popup/heads up notifications globally in Android 9 Pie?
My S9+ is running Androud 10 UI2 and Telegram 6.0.1
I had the same issue which I finally solved. I went all through the app settings, then i tried android-apps-telegram settings with no luck.
I finally looked in android-settings-notifications and down at the bottom it says "looking for something else?" Click on the Smart pop-up view option and shut telegram off.