WebExtension: browser_action icon in incognito mode

my webextension browser_action icon is correctly displayed in a normal window.
but it is not displayed in “Private Browsing” windows…

how can I make the icon appear also in Private Browsing mode ?

manifest.json:

Riepilogo

{

  "manifest_version": 2,
  "name": "xyz",
  "version": "1.0",

  "applications": {
    "gecko": {
      "id": "xyz@xyz",
      "strict_min_version": "54.0"
    }
  },

  "icons": {
    "128": "icons/icon-128.png"
  },

  "browser_action": {
    "default_icon": {
      "32": "icons/icon-32.png",
      "64": "icons/icon-64.png"
    }
  },

  "permissions": [
    "tabs", "webRequest", "webRequestBlocking", "webNavigation", "*://*.xyz.com/*"
  ],

  "background": {
    "scripts": ["js/xyz.js", "js/xyz2.js"]
  },

  "content_scripts": [{
    "matches": ["*://*.xyz.com/"],
    "js": ["js/xyz3.js"],
    "run_at": "document_start"
  }]
}

Go to about:addons (the Add-on manager), click on your extension and then allow it to run in private browsing.