Below, I will create a fence of length 4, followed by a new line, followed by three tildes, followed by a new line, followed by a fence of length 4, which should create a code block with three tildes in it, but instead:

Correct, because the standard is still provisional... We did try to make markdown.js, which is what is used here, pass MDTest 1.1 and submitted many upstream patches to make that happen.