I would send feedback to the Skype devs about this.
It almost sounds like the app is turning off the screen due to the proximity sensor detecting something near it (which is how a phone turns off the screen during a call when it's held up to the ear). I'm not sure if the 2020 Tab M8 has a proximity sensor, but if it does, then something blocking that sensor (like a poorly fitted screen protector, or even some grime overlying the sensor) might trigger that. The sensor would be right near the front-facing camera, so make sure that area is completely clear, and not blocked by a screen protector (even if that protector is clear).