monaco-editor version: 0.17.0
Browser: Edge (Chromium) Version 77.0.189.3 (Official build) dev (64-bit)
OS: Windows 10

Steps or JS usage snippet reproducing the issue:

1. add handler for onDidChangeConfiguration
2. on editor surface, press Ctrl+M to toggle tab focus mode change
3. event is raised and IConfigurationChangedEvent.tabFocusMode will be true, as expected
4. on editor surface, press Ctrl+M again to toggle tab focus mode (back to false)
5. IConfigurationChangedEvent.tabFocusMode is still true

issue can be reproduced in the playground.

var editor = monaco.editor.create(document.getElementById("container"), {
	value: "function hello() {\n\talert('Hello world!');\n}",
	language: "javascript"
});

editor.onDidChangeConfiguration((e) => {
    alert("Tab focus mode is " + e.tabFocusMode);
});