Apple - Why isn't Alt-Tab working for switching windows of the same app?

Try Command + ` (backtick). That is the default.

Option+Tab shouldn't be assigned to switching windows by default. You could've reassigned it in the Keyboard preferences:

It could also be handled by some third party app like Witch, LiteSwitch X, or Keyboard Maestro on the MacBook Pro.

Use Hyperswitch. I just stumbled on this posting now, and did some digging and found this nice "new" app.

https://bahoom.com/hyperswitch

Works well for me (basically turns your mac's application-switching into a windows-style task-switching environment <- much more intuitive especially when working w/ multiple screens/spaces).