Visual Studio Add-in launches wrong version of designer.exe

I have two Qt versions configured, 4.7.2x64 and 4.7.3x64. When I click the Visual Studio menu item Qt>Launch Designer, I expect it to launch designer.exe from the default version's bin folder. However it always launches the designer.exe from 4.7.2x64/bin, no matter which is set to be the default. To be able to run the designer.exe in 4.7.3x64, it seems I have to delete or rename the one in 4.7.2x64, then somehow it magically finds the right one!

Note: I first thought this was an environment issue, but neither 4.7.2x64/bin or 4.7.3x64/bin are in %PATH% and I can see in the task manager that the command is invoked with the full path.