The 2 methods are different:

std::string::find_last_of is used to:

Finds the last character equal to one of characters in the given character sequence.

(emphasis is mine)

Whereas std::string::rfind is used to find a given substring (not just one of its characters):

Also note that with std::string::rfind the count parameter determines the length (i.e. number of characters) of the substring to search for:

count – length of substring to search for

Therefore in order to find a string of 1 space character, you need to pass 1 for count.
std::string::rfind also has an overload that does not require count and simply serached for the substring as given.
In your case you can also use a 3rd overload that does not require a position and simply searches the substring from the right.

#include <string>
#include <iostream>

int main() {
    std::string s = "a abc test";
    //---------------------------v---------
    std::cout << s.rfind(" ", 5, 1) << "n";        // prints 5
    std::cout << s.rfind(" ", 5) << "n";           // prints 5
    std::cout << s.rfind(" ") << "n";              // prints 5
}

Output:

5
5
5

Live demo

(Note: the live demo is with gcc because Godbolt does not support an executor for MSVC at the moment, but I get the same result with VS2022).

A side note:
Your usage of count in find_last_of is also based on incorrect interpretation of the parameter (despite the fact that you got the expected result). It actually refers to the “length of character string identifying characters to search for”, and not to the source string where the search is done.
And BTW – if you passed 3 or more as count it was also triggering UB (undefined behavior), because your s is " " which is char[2]:

If [s, s + count) is not a valid range, the behavior is undefined.

The count, third parameter in std::string::rfind 2) is the number of chars in the substring, first parameter. 4 is an invalid number for " ". 2 is valid and will search for that cannot be found in a abc test

#include <iostream>
#include <string>

int main() {
  const std::string s = "a abc test";
  std::cout << s.find_last_of(" ", 5, 2) << "n";
  std::cout << s.rfind(" ", 5) << "n";
  std::cout << s.rfind(" ", 3) << "n";
}

Outputs

5
5
1

The first statement also does not do what you want. 2 sets the chars to search ' ' and '' from the string literal " ". You just don’t have '' in "a abc test"s.