Steps to reproduce:
1. Disable TinyMCE in your Moodle profile at moodle.org
2. Compose a forum post in Markdown format, eg:
3. Save the form
4. Edit the post
The text should appear in the textarea exactly as you typed it (following the "What You See Is What You Typed" principle). This has always been like that, 10 of 10 Markdown users say.
What actually happens:
The > character is displayed as > entity, breaking the Markdown syntax.
I am not sure if the same affects other formats, too. Please try HTML format, too.