We have requirement for adding Favicon for Lightning Home page which was built in Lightning App Builder. If the page is in Tab or Preview mode I am able to see favicon, but the samepage is in Lightning App Builder I am able to see salesforce default favicon.
When posting browser specific issues, make sure you include the following:
-device being used
Validate that you are using the latest browser version and see if you can reproduce the issue, test with other device(s) as well.
If the issue persists, contact browser support, for ex.
for google chrome:
Or last but not least, Salesforce support. unfortunately, there isint much we can do here for these specific issues.