Why is the execution order different in addition and subtraction?

Question

I have a function that takes one argument by address and changes the value of the argument. But when I call the function, the order of execution is different in cases of addition and subtraction:

#include <iostream>
using namespace std;

int fun(int *i) {
    *i += 5;
    return 4;
}

int main()
{
    int x = 3;
    x = x + fun(&x);
    cout << "x: " << x << endl;
    
    int y = 3;
    y = y - fun(&y);
    cout << "y: " << y << endl;

    return 0;
}

The output of the above code is:

x: 12

y: -1

which means x is first changed in the function and then added to itself (I expected x to become 7), but the changes on y don't affect the y before subtraction operator.

Why is this happening?