• VSCode Version: 1.36.1
• OS Version: Windows_NT x64 6.1.7601

Steps to Reproduce:

1. Add the following to user config to disable line numbers in markdown files

"[markdown]": {
    "editor.lineNumbers": "off"
},

2. Open a markdown file, note that line numbers are disabled as expected
3. Activate Zen mode
4. Deactivate Zen mode
5. Line numbers reappear despite being disabled for markdown files in config

Note that this only happens if you disable line numbers for a specific mode. If line numbers are disabled globally everything works as expected and they never reappear.

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