!Tab::returnTurns out the script sends ALT+CTRL when the ALT+TAB hotkey is triggered. Tested with other ALT+key as hotkey, CTRL is sent in most combinations. Tested it out with http://en.key-test.ru/ and http://www.keyboardtester.com/tester.html, both URLs are to online keyboard tester. Someone else from the Discord server also tested and report that it also happens on their PC.
My details: AHK 1.1.26.01 64-bit, Windows 10 creators update