On the new Suggested Edits page, I saw an edit earlier that did not have the "markdown diffs" toggle available:
==> I moved to the next edit and it did have the option available:
- Is it a bug?
- If not why is there a difference?
The title is a plain text field; no markdown is interpreted in it. As such, there is no point in showing a markdown diff.
You'll see the same (no diff options) when reviewing suggested edits for tag excerpts, and when only the tags on a question have been changed.