Having a similar issue (Amazing app by the way).
I've imported a .md file with fenced code blocks and it sees the first 2 back ticks as a pair inline code backticks. Then, as I was messing around trying to find the symbol in the iOS keyboard, somehow I got the fenced code block to start working.
However, I haven't been able to replicate this behaviour again and the preview still shows the working fenced code block as it being all on one line. And I've never had syntax hilighting in 'preview' - is this normal?
Edit: Seems like there was a leading space before the code block start that stopped it triggering. Even after it triggered it was still rendered as one line and not hilighted.
Hope this doesn't sound like a nag post, just thought id contribute to a small flaw in an almost perfect app.