I am able to reproduce this with iOS 8. It is iPhone 4S but I expect the same result on other devices too. There is nothing in packaging logs about this issue, but I can provide if you still request it.
I prepared a test page for this.
https://www.skateandstrike.com/test/get.php?mytext=Hello%20World
Could you please try with this link? In Android and Windows, it is going to the correct page:
https://www.skateandstrike.com/test/get.php?mytext=Hello%20World
But if I use same function in iOS, it goes to this page:
https://www.skateandstrike.com/test/get.php%3Fmytext=Hello%20World
I think problem is in the iOS specific function that is defined here:
Engine\Source\Runtime\Core\Private\IOS\IOSPlatformProcess.cpp\FIOSPlatformProcess::LaunchURL
There is a percent encode computation here. I think problem is at that computation.