macOS 27 does not return to last active app window

Your understanding of ⌘Tab is correct.

Apple documents ⌘Tab as switching to the next most recently used app, while ⌘` switches between windows of the current app. (Apple Support)

First, make sure Window A is still open and not minimized when you switch away from it. Then test with two windows of the same application in the same Space, with Stage Manager off. 

Make Window A active, switch to another application with ⌘Tab, and return to the original application with ⌘Tab. Repeat the test with Finder. Also try clicking the application's Dock icon instead of using ⌘Tab.

If you use Stage Manager, check System Settings >> Desktop & Dock >> Stage Manager >> Customize >> Show windows from an application. 

Apple provides One at a Time, which shows only the most recently used window when switching to an app. This setting applies specifically to Stage Manager. (Apple Support)

If you use multiple Spaces, check System Settings → Desktop & Dock → Mission Control → “When switching to an application, switch to a Space with open windows for the application.” This controls whether macOS moves to a Space containing windows for the selected application. (Apple Support)

If the behaviour occurs in several applications, including Finder, repeat the test in a temporary macOS user account. If the behaviour changes there, something associated with the original user account may be contributing to it.

Because you first noticed the behaviour after upgrading from macOS 26 to macOS 27, record whether Window A remains open and unminimized, which applications are affected, and whether the behaviour changes with Stage Manager, Spaces, the Dock, or a different user account. 

Those results provide a more useful basis for narrowing down the cause without assuming that the behaviour is necessarily a macOS 27 regression.

Apple: Mac keyboard shortcuts · Apple: Desktop & Dock settings · Apple: Stage Manager · Apple: Move and arrange app windows

Yes — the Stage Manager toggle is a useful workaround to report here.

If turning Stage Manager on and then back off restored the expected ⌘Tab behaviour, that suggests the problem was likely related to macOS's window-management state rather than a Firefox or VS Code setting.

A concise way to describe the workaround would be:

Workaround: Nothing else I tried fixed it, but turning Stage Manager on, then turning it off again, appears to have resolved the problem. ⌘Tab now returns me to the specific window I was last using in the app, as expected. I haven't changed any other settings, so I can't say exactly what caused the issue or why toggling Stage Manager fixed it. If you're experiencing the same problem after upgrading to macOS 27, it's worth trying this before changing your ⌘Tab or window-switching settings.

I would avoid claiming that Stage Manager causes or fixes the underlying bug, since your experience establishes that the toggle restored the behaviour, but not why.

Nothing helped, but after turning on then off stage manager seems to have resolved the issue.

Strange

What you are describing is specifically a problem with which window becomes active when you return to an application with ⌘Tab.

You are not asking how to switch between an application's windows with ⌘, and changing the ⌘Tab/⌘ shortcuts does not address the issue.

You have reproduced it in Firefox and VS Code, and the behaviour was different for you on macOS 26. Turning Stage Manager on and then off appeared to help temporarily, but the problem returned, and double-clicking a title bar has also been inconsistent.

Apple's documentation confirms that ⌘Tab switches applications, but it does not provide a setting specifically controlling which window is restored when an application is selected. The Mission Control setting concerning switching to a Space with open windows is related to Spaces, but it does not specify which window within the application should become active.

I therefore would not recommend changing additional macOS settings based on the information currently available. The appropriate next step is to determine whether other macOS 27 users are reporting this exact behavior and whether Apple or the affected applications have identified a cause or reliable workaround.

Apple: Mac keyboard shortcuts

Apple: Change Desktop & Dock settings on Mac

Nah **** ain't fixed, but double clicking title bar for some windows unreliably fixes it

The other clanker here is just FUD