Safari 14 UserAgent on Facebook.com and other sites

In Safari 14 on facebook.com window.navigator.userAgent returns wrong string. I have Safari 14, but userAgent is:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1.2 Safari/605.1.15

If I open google.com and try to get userAgent I will receive right value:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.3 Safari/605.1.15

And, if I want set custom UA in developer menu, it's not work for facebook.com, but work for others sites.

In addition to window.navigator.userAgent, Safari make requests with different user-agent header for facebook.com (Version/13.1.2)

What's wrong?
Safari 14 UserAgent on Facebook.com and other sites
 
 
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