После npm run build на хостинге во вкладке нету favicon.ico. В папке public находятся:
index.html <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<link rel="shortcut icon" href="./favicon.ico">
<meta name="theme-color" content="#000000">
<script type="text/javascript" src="https://cdn.emailjs.com/sdk/2.1.0/email.min.js"></script>
<link rel="manifest" href="./manifest.json" />
<link rel="stylesheet" href="//cdn.jsdelivr.net/npm/semantic-ui@2.4.2/dist/semantic.min.css"/>
<title>Jay - лёгкость в каждом движении</title>
</head>
<body>
<noscript>You need to enable JavaScript to run this app.</noscript>
<div id="root"></div>
</body>
</html>
manifest.json{
"short_name": "React app",
"name": "React app",
"icons": [
{
"src": "favicon.ico",
"sizes": "64x64 32x32 24x24 16x16",
"type": "image/x-icon"
}
],
"start_url": ".",
"display": "standalone",
"theme_color": "#000000",
"background_color": "#ffffff"
}
и
favicon.ico 
Средний
