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[BUG] IntersectionObserver not polyfilled on iOS device

See original GitHub issue

What

I am receiving reports of the following error:

Can't find variable: IntersectionObserver

My polyfill request looks like this: https://polyfill.io/v3/polyfill.js?unknown=polyfill&features=default%2CSymbol%2Cfetch%2CArray.prototype.find%2CIntersectionObserver%2CWeakSet%2Ces2015%2Ces2016%2Ces2017&flags=gated

The user agent string: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/12.1.2 Safari/605.1.15

When I call the service with my user agent string spoofed I am receiving the following header in the javascript file:

/* Polyfill service v3.38.0
 * For detailed credits and licence information see https://github.com/financial-times/polyfill-service.
 * 
 * Features requested: Array.prototype.find,IntersectionObserver,Symbol,WeakSet,default,es2015,es2016,es2017,fetch
 * 
 * - _ESAbstract.Call, License: CC0 (required by "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015", "_ESAbstract.OrdinaryToPrimitive")
 * - _ESAbstract.CreateMethodProperty, License: CC0 (required by "Math.clz32", "es2015", "Math.sign", "String.prototype.repeat")
 * - _ESAbstract.Get, License: CC0 (required by "_ESAbstract.OrdinaryToPrimitive", "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.IsCallable, License: CC0 (required by "_ESAbstract.GetMethod", "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015", "_ESAbstract.OrdinaryToPrimitive")
 * - _ESAbstract.RequireObjectCoercible, License: CC0 (required by "String.prototype.repeat", "es2015")
 * - _ESAbstract.ToInteger, License: CC0 (required by "String.prototype.repeat", "es2015")
 * - _ESAbstract.ToObject, License: CC0 (required by "_ESAbstract.GetV", "_ESAbstract.GetMethod", "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.GetV, License: CC0 (required by "_ESAbstract.GetMethod", "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.GetMethod, License: CC0 (required by "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.ToUint32, License: CC0 (required by "Math.clz32", "es2015")
 * - _ESAbstract.Type, License: CC0 (required by "_ESAbstract.ToString", "String.prototype.repeat", "es2015", "_ESAbstract.ToPrimitive", "_ESAbstract.OrdinaryToPrimitive")
 * - _ESAbstract.OrdinaryToPrimitive, License: CC0 (required by "_ESAbstract.ToPrimitive", "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.ToPrimitive, License: CC0 (required by "_ESAbstract.ToString", "String.prototype.repeat", "es2015")
 * - _ESAbstract.ToString, License: CC0 (required by "String.prototype.repeat", "es2015")
 * - Math.clz32, License: CC0 (required by "es2015")
 * - Math.sign, License: CC0 (required by "es2015")
 * - String.prototype.repeat, License: CC0 (required by "es2015") */

A search in the rest of the file however shows no sign of IntersectionObserver.