hebrewbooks has a scan of the 1558 Mantuba edition of Tikunei HaZohar. In that edition, Tikun 20 is several pages long. (Starts here, goes for several pages).
The only other edition I found on hebrewbooks is a far more modern (1960s) edition of tikunei haZohar. In it tikun 20 is one line long. This is also repeated in the text of ha-zohar.com where tikun 20 is abbreviated and combined with 21. This is repeated on Torat Emet, wikisource, and many other places. While many of the internet sites are clearly copying from each other, there is a printed book and a digital copy (original website unknown) that have the abbreviated tikun 20. What happened to it? Was this dropped deliberately or is something else going on?