Foldtext not making any difference to text displayed in closed fold

Question

I changed the value of the foldtext option to change the text displayed in a fold, and it made no difference.

I have a short file in which I defined a manual fold containing two lines, with the command: zf2j. The closed fold displays the following text: +-- 2 lines folded.

I put in my .vimrc the following:

set foldtext=MyFoldText()
function MyFoldText()
  return "wtf?"
endfunction

I then entered :e in the testfile to reload the config, and nothing changed. The closed fold still displays: +-- 2 lines folded.

Entering :set foldmethod?, tells me manual. Entering :set foldtext? tells me MyFoldText().

I tried changing it directly, by entering in the testfile the command: :set foldtext=v:folddashes. Nothing changed. The closed fold still displays: +-- 2 lines folded.

What am I doing wrong?