Fundamentally - we didn't have code fences then. We do now. Essentially - the conversion from markdown to rendered markup happens once - when the post is posted.
For extra fun - since its an odd number of backticks, the first pair in the opening code fence was "one" set of unformulated characters, the last and the first of the closing pair, and the 2 at the end each acted as a pair of backticks. So essentially, we were wrapping all that in one backtick
And the edit window was crunching the markdown on the fly to show you what you would see, so it ignored the obsolete markdown rendering
A single character unrelated edit caused the system to crunch the markdown, and all (the formatting) is good again