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Safari on iOS 14 is being detected as iOS 11

See original GitHub issue

What

We have noticed that when using Safari on an iPhone 12 on iOS 14, fetch (among other things) is being polyfilled when it doesn’t need to. This has caused some unexpected issues with some DataDog client-side JavaScript we are using.

Details

To reproduce

Using Chrome’s dev tools, set the User Agent String to: Mozilla/5.0 (iPhone; CPU iPhone OS 14_3 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.1 Mobile/15E148 Safari/604.1
Go to the following polyfil.io URL: https://polyfill.io/v3/polyfill.min.js?features=es2017%2Ces2016%2Ces2015%2Cdefault%2CArray.prototype.contains%2CDOMTokenList.prototype.%40%40iterator%2CDocumentFragment%2CEventSource%2CFunction.name%2CIntersectionObserver%2CMutationObserver%2CNodeList.prototype.%40%40iterator%2CObject.freeze%2CPromise.prototype.finally%2CString.fromCodePoint%2CString.prototype.contains%2CUserTiming%2CWebAnimations%2Cfetch%2ClocalStorage%2CmatchMedia%2Cnavigator.sendBeacon%2Cperformance.now%2Cscreen.orientation%2CsetImmediate%2CResizeObserver%2CAbortController%2CIntl.PluralRules,Intl.PluralRules.~locale.en
Go to the Network Tab in Dev Tools.

This can also be reproduced simulating an iPhone 12 on iOS 14 using Simulator on a Mac or using BrowserStack.

Expected result

The response should not contain any code for polyfilling fetch
The detected-user-agent response header should have a value like Mobile Safari/WKWebView/14..
The normalized-user-agent response header should have a value like ios_saf/14.0.3

Actual result

The response contains code for polyfilling fetch
The detected-user-agent response header has the value Mobile Safari/WKWebView/11..
The normalized-user-agent response header has a value of ios_saf/11.0.0

The following screen grab shows the above result:

Additional information

I believe the good work done in https://github.com/Financial-Times/useragent_parser/pull/101 should have fixed it, and when running the service locally it works as expected. Is it possible that there’s a cache that needs clearing?