The format that I like to use for bug reports (that I write myself, and that I like to receive) is as follows:

• Summary/title (one-line at most)
• Steps to reproduce
• Expected behavior
• Actual behavior

Explicitly mentioning the expected and the actual behavior is useful to avoid any assumptions about what is expected behavior.

Optionally, provide context:

• OS version
• Interpreter version
• Package manager version
• Stack trace (if regarding a crash)