When you use ALT+TAB to switch to a different application, that application gets it's focus set to it's menu. This is highly undesirable because you have to press ESC immediately after switching to the other application to prevent accidentally giving it commands when you were only wanted to type something in a text field. Quickly switching applications while working is a common and frequent use case that's completely broken by this new behaviour.
@Gerard_V I experience the very same thing. Every single time when switching from one application to another using ALT+TAB, I have to mind not to start typing e.g., as the menu of that application is then highlighted with the shortcut function/alt function. Really annoying! So far, I haven't found a solution for this unwanted behaviour yet.