Whatsapp call echo when screen is turned off

I've noticed a weird thing happening on my phone.

When I'm on speaker on a Whatsapp call and turn off the screen using the power button, the user on the other end hears an echo.

Strangely, it goes away when I turn on the screen and turn it off using the same power button.

This has happened almost every single time and I wonder if it's happening to everyone else?