Not sure if this is your exact problem but there's a bug somewhere between MS and Apple – when they updated Safari on iOS8 they bumped the version to be Webkit 600, but the code in .NET that’s looking for the browser capabilities screws up and only looks at the first two numbers, so it’s seeing “Webkit 60” instead. The Ajax controls therefore don’t think the browser supports the callbacks and falls back to not using it.
Fix is relatively simple – we need to override the browser capabilities in the web.config – add this to System.Web
<browsercaps>
<filter>
<case match="AppleWebKit">
EcmaScriptVersion = 1.5
supportsCallback = true
</case>
</filter>
</browsercaps>
This isn't the only solution, but was the most expedient for us when we encountered a similar problem earlier this year.
References:
http://www.brillianceweb.com/blog/article/69/aspnet-20-misidentifies-safari-71-on-ios-and-os-x#sthash.0FuAyobK.dpbs