Pasting certain characters in a pull request description wraps all pasted text in a code block

Steps to reproduce:

1. Open a pull request
2. Paste the attached text into the description

Outcome:

• All pasted text is now in a code block

Expectation:

• The first line is not in a code block
• The lines surrounded by three backticks are in a code block

Notes:

• The bug is very inconsistent. Changing one letter from the example makes it go away.
• The bug isn't limited to this simple example. I originally found this when I pasted in some JSON.