This happens only in Skype on Ubuntu 18.04. I have used Skype on other platforms (Ubuntu and others) and this does not happen. I use ALT+SHIFT to switch keyboard layout to another language. When I use Skype on Ubuntu 18.04 and press ALT+SHIFT the language is changed but also the focus is moved to the application menu (as if I had pressed ALT only).
Any help is welcome!