ImageSource bug

Description

Sometimes the Xamarin.Forms.Image does not display image from the Xamarin.Forms.ImageSource.

Steps to Reproduce

Sample: https://yadi.sk/d/q_TXwro3Fti_sA Alternately tapping on the top images: left, right, left, right... Sometimes you can see 3 images: https://yadi.sk/i/_8kjsQO1UIwRjQ

If add breakpoint for System.Exception for Visual Studio for Mac, you catch exception: System.Threading.Tasks.TaskCanceledException

StackTrace:

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00020] in /Library/Frameworks/Xamarin.iOS.framework/Versions/12.0.0.15/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:179 
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x00028] in /Library/Frameworks/Xamarin.iOS.framework/Versions/12.0.0.15/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 
  at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x00008] in /Library/Frameworks/Xamarin.iOS.framework/Versions/12.0.0.15/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 
  at System.Runtime.CompilerServices.TaskAwaiter`1[TResult].GetResult () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/12.0.0.15/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:357 
  at Xamarin.Forms.StreamImageSource+<Xamarin-Forms-IStreamImageSource-GetStreamAsync>d__5.MoveNext () [0x00066] in D:\a\1\s\Xamarin.Forms.Core\StreamImageSource.cs:36

Expected Behavior

After first tap(and more tap) you must always see 4 images: https://yadi.sk/i/e1nzEY45kuepKw

Actual Behavior

After first tap(and more tap), sometimes you may see 3 images: https://yadi.sk/i/_8kjsQO1UIwRjQ

Basic Information

• Version with issue: Xamarin.Forms(3.3.0.967583)
• Last known good version: no
• IDE: Visual Studio for Mac Community 7.6.10 (build 27)
• Platform Target Frameworks:
  • iOS: 12.0
  • Android: 8.1
• Nuget Packages:
• Affected Devices: iPhone5, iPhone7 and emulators for iOS and Android

Screenshots

See above

Reproduction Link

Sample: https://yadi.sk/d/q_TXwro3Fti_sA