This has to do with the age of the post and is in fact happening by design. When it was created in December 2018, Stack Exchange didn't support
``` code fences. It did, however, support
``` as a way of entering inline code with like breaks. That's what the answer in question was doing, purely by accident.
In January 2019, we added support for code blocks as described here: https://meta.stackexchange.com/a/322000/235711
Since Markdown is rendered at time of edit, the answer continued to look off until it received its first edit after January 2019. Since the editor and renderer now support code blocks, the preview correctly matched what would be saved, even though there was no actual change to the markdown.