It’s a confusing example, but it does accord with the spec, so it’s not a bug. First, remember that as the spec states explicitly, markers of block-structure take precedence over markers of inline structure. The line
```. The preceding code is inline with this text.
meets the spec’s criterion for opening a fenced code block. (The first line does not, because info strings are not allowed to contain backticks.) Because this line opens a code block, it isn’t parsed as part of the preceding paragraph, and so its triple backticks can’t close the triple backticks in the preceding paragraph, just as they wouldn’t in this case:
``` some `code`
```
(Recall that fenced code blocks can interrupt paragraphs and don’t need a preceding blank line. Recall also that a fenced code block in commonmark does not need a closing fence; if no closing fence is encountered, it extends to the end of the document.)