I've done some testing.
The "tokenat" function seems to recognize "my string" alone as a separator and also recognizes a Newline alone as a separator, but it doesn't seem to recognize "my string" & newline as a separator.
I should add that "my string" & newline do follow each other directly such that they should work as a separator.
Also, I tried using the "find" function on my source string to look for the index of "my string" & newline. Unfortunately it returns a -1. I do get the correct index if I use "find" with "my string" or Newline, just not when I use "find" with "my string" & newline.