Filenotfoundexception in visualstudio IDE when multilevel DLL's are referenced

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VB

WinForms

Hello,
my environment is :
- Windows 7, VS 2010 SP1, FW .NET 4.0.

The issue is :
- I have a DLL .NET (dll1.dll version 1.0.0.0 without StrongName) who contains a simple class that does anythis ;
- I have a second DLL .NET (dll2.dll version 1.0.0.0 without StrongName) who contains a WinForms UserControl, with one simple Button control ; dll1 is also referenced, with SpecificVersion property set to false ;
- I have a WinForms application, who contains just one Form. dll1 and dll2 are referenced with SpecificVersion property set to false, and in the VS IDE, I add the UserControl in the ToolBox, then I drag and drop one instance of UserControl on the Form ;

At this point, application compilation (x86) and runtime are OK.

Then :
- I modify a little bit code in dll1, I modify the version of dll1 from 1.0.0.0 to 1.1.0.0, and I recompile dll1 ;
- BUT I DON'T WANT to recompile dll2 because his code has no changes ;
- In the WinForms application, reference to dll1 is upgraded to 1.1.0.0, and reference to dll2 is still to 1.0.0.0.

Results :
- compilation is still OK ;
- runtime is still OK, because there is no StrongName ;
- BUT when I try to display the Form in the IDE, VS throws a FileNotFoundException because it cannot found "dll1 version 1.0.0.0".

I don't understand why, at design time, VS throws this exception that is not thrown at runtime.

I hope that my explanations are clear and detailed enough.

Thank you for your help
Chris

What I have tried:

With VisualStudio 2013, same issue.