When Marky is fed an HTML fragment that includes the & (ampersand) char, it will choke/stop processing.
While testing my @workflowhq workflow Emulate Drafts Web Capture Keeping Links, I noticed that if the clipboard has an "&" char, the MMD output I get is truncated there. Further testing validated that if the html fragment that is fed to Marky via the "HTML" option (http://heckyesmarkdown.com/go/?html={fragment}&read=1&md=1) contains an ampersand processing stops there.
I've built a test case to show the problem.
HTML Fragment:
<html>
<body>
Testing Ampersand & Marky behavior with HTML fragments
</body>
</html>
Resulting Marky URL: http://heckyesmarkdown.com/go/?html=%3Chtml%3E%0A%3Cbody%3E%0ATesting%20Ampersand%20&%20Marky%20behavior%20with%20HTML%20fragments%0A%3C/body%3E%0A%3C/html%3E&read=1&md=1
Test case Workflow: For ease of testing I've built a demo Workflow (https://workflow.is/workflows/2a381048a30049cf8bd2b5554ccffe5d) that takes the HTML code in a Text action, runs it through Marky and creates a Drafts4 note with the input, URL and result.