That is true, please look at the example in the kata's description. For the case you say it can't be true, the explanation would be:

'acab' is a merge from 'aa' and 'cb':

    s:  a c a b   = acab
part1:  a   a     = aa
part2:    c   b   = cb

Invalid result for s = "acab", part1 = "aa", part2 = "cb" should be false but says its true this cant be true beacuse we dont have aa or cb there

There is nothing to fix, you can't reorder the letters, read the description again.

Invalid result for s = "acab", part1 = "ca", part2 = "ba" it should be false it says but actually should be true please fix this also with Invalid result for s = "aaaabaaaab", part1 = "aaaba", part2 = "aaaba" this should be true too but says false.

u can use
std::transform(temp.begin(), temp.end(), temp.begin(), ::tolower);
so u dont have to use two different variables for lower and uppercase.

based, sigma , gigachad.