I have also seen william lin solve it using hash_map in his cses speedrun video on youtube.
But C++ STL unordered_map is a 1 to 1 mapping of key and value. So say x = 12, Now we could make x by doing (a1+a4) + (a2+a3) = 5 + 7. But let's say (a3+a4) = 7 as well , and as unordered_map records only the latest occurrence of 7, so we only have 7 at (a3+a4). As a4 is repeating in both (a1+a4) and (a3+a4), these are not distinct positions and we don't have an answer.
This is not a perfect example, because we can discover other good ways to make 12 if we check other possible combinations. And also a perfect example might not exist given that this is a well-known solution, I just wanted convey my idea why unordered_map might fail and it is not all clear to me why it actually works for every possibility.
