I'm not sure if I've made an error, or if this is a bug in the Backporter script.
Look at this PR (JuliaLang/julia#59521), and in the PR description, look at the edit history, and in particular compare my first edit to my second edit. Here are some screenshots.
My first edit:
 
My second edit:
 
In both cases, I have simply copy-pasted the contents from the Backporter script's output.
In my first edit, you can see that #59300 is listed there under "Backported PRs".
In my second edit, for some reason  #59300 has been removed from the list. But if you look at the PR commits (https://github.com/JuliaLang/julia/pull/59521/commits), you'll see that #59300 is still backported.