Description:
Reproduction project demonstrates this issue with Sockets. The symptoms are the same for Dns, Sockets and HttpWebRequest. When you run "Scene.unity", it will start 120 requests to "www.google.com:81 (port that drops all packets)", and will start cancelling them after 3 seconds. If you keep it running for about 1 minute, you should see "[End] : ..." in the console 120 times. That "[End] : ..." log is written inside of the socket's callback.

Reproduction steps:
1. Open the attached project: "02/21/2017 6:06 PM"
2. Build and run the "scene" for iOS device.
3. Wait for the application to call the 120 end calls.
4. Check the every (0 - 119) "[End] :" line in the Xcode console.

Actual behavior:
Some of them are missing.

Expected behavior:
Every "[Start]:" has the ""[End] :" pair.

Reproduced with:
iPhone 6+ 8.0
5.3.8f1, 2017.1.0b2

Not reproducible with:
Android

Notes:
Android log attached in the edit: 874290_GooglePixel_560p1
iOS log attached in the edit: log-iphone-6-Plus-iOS-8.0