"PREF: remove default Top Sites" doesn't work

Prerequisites

• The issue persists when using Firefox in Troubleshoot Mode.
• The issue persists after I disabled DNS-level content blocking (PiHole, NextDNS, etc.), cleared DNS cache, and tried to use ISP's DNS resolver (if applicable).
• The issue persists after I cleared website data and restarted Firefox.
• I have created a new, clean profile. The issue persists after adding the user.js, but it is not reproducible in a new, unedited Firefox profile.

Bug description

Hello everyone. It seems that setting browser.newtabpage.activity-stream.default.sites to empty string does not clear default Top Sites by itself, because Firefox fetches them from elsewhere. However if you also set browser.topsites.useRemoteSetting to false it does clear them.

Steps to reproduce

Prerequisites: Use both "hardening" and "override" prefs in your user.js.

// PREF: hide site shortcut thumbnails on New Tab page
user_pref("browser.newtabpage.activity-stream.feeds.topsites", false);

// PREF: hide weather on New Tab page
user_pref("browser.newtabpage.activity-stream.showWeather", false);

// PREF: hide dropdown suggestions when clicking on the address bar
user_pref("browser.urlbar.suggest.topsites", false);

// PREF: restore Top Sites on New Tab page
user_pref("browser.newtabpage.activity-stream.feeds.topsites", true);

// PREF: remove default Top Sites (Facebook, Twitter, etc.)
// This does not block you from adding your own.
user_pref("browser.newtabpage.activity-stream.default.sites", "");

// PREF: remove sponsored content on New Tab page
user_pref("browser.newtabpage.activity-stream.showSponsoredTopSites", false); // Sponsored shortcuts 
user_pref("browser.newtabpage.activity-stream.feeds.section.topstories", false); // Recommended by Pocket
user_pref("browser.newtabpage.activity-stream.showSponsored", false); // Sponsored Stories

1. Go to about:config and set browser.newtabpage.activity-stream.feeds.topsites to true. On about:newtab default Top Sites will pop up, even when browser.newtabpage.activity-stream.default.sites is set to empty string.
2. Set browser.topsites.useRemoteSetting to false and default Top Sites disappear.

Expected behavior

No response

Screenshots

No response

Operating System (OS)

Windows (10 or later)

Browser

Firefox Developer Edition

Version

156.0b5

Additional information

No response