I tested in older builds of Opera and see that typing a search engine shortcut like 'a' (when set for Amazon for example) just by itself does nothing and just searches for 'a' with the default search engine. A space after the 'a' though does the trick and searches Amazon (for example) with no search term.
In the latest builds of Opera, I see that 'a' and a space for example no longer does the trick and it has to be 'a', a space and then some other character that's not a space.
I'm not sure if that's an intentional change or not and not sure if the change comes from Chromium or something Opera did.