Given that Markdown has lots of undefined semantics, this may very well be a valid interpretation of certain broken Markdown documents. But I guess most would agree that the priority should be the other way around.
Example: (Save as `something.md` and open with Geany)
_This is "underlined"
``` should_be code ```
This sentence is considered code, because the "matching" underscores had priority (?)
Note that the "stray underline" can also occur in HTML inline-comments, where they don't stand for underlining anyway.
<!-- Like this one. I know very well that this won't usually appear. -->