Does this issue occur when all extensions are disabled?: Yes

Version: 1.64.0-insider
Commit: 2366435
Date: 2022-01-21T13:26:52.572Z
Electron: 13.5.2
Chromium: 91.0.4472.164
Node.js: 14.16.0
V8: 9.1.269.39-electron.0
OS: Linux x64 5.13.0-27-generic

Steps to Reproduce

1. Open the microsoft/vscode repository
2. In the Search sidebar, click "Open a New Search Editor"
3. Enter a multi-line search query:

## Asking Questions

Have a question?

4. In the results, blank lines are replaced with the word undefined. (Only blank lines within the matched range are affected; blank lines in the surrounding context are rendered correctly.)

CONTRIBUTING.md:
  6  
  7: ## Asking Questions
  8: undefined
  9: Have a question? Rather than opening an issue, please ask away on [Stack Overflow](https://stackoverflow.com/questions/tagged/vscode) using the tag `vscode`.
  10