as rowcased said, your code contains undefined behavior. you are accessing the vector past its last element. one property of undefined behavior is that it can cause code to work in one setting (your computer) and not in another (Codewars). this is not a kata issue, but a problem with your code.

It seems strange, because on my VS Code with compiler gcc 8.3.0 i have correct answer for {4,16,24} and {30,12}

I am not even sure this is possible, but it appears that, during a particular test, using arr[i+1] within a loop based on i < arr.size() you end up causing UB by altering the elements of the following test's arr1?

actual refers to the value your code returns to the testing suite

so, your code is returning 4 for that test, whereas it should return 12 for that test

I have the problem with basic tests. In results actual value is 4, but 4 is not in tests.