The views should be in .Views child namespace in order to get it working as expected, as explained in this article
from the Prism documenation in MSDN:
This convention assumes that view models are in the same assembly as the view types, that view models are in a .ViewModels child namespace, that views are in a .Views child namespace, and that view model names correspond with view names and end with "ViewModel.".
Hope this helps.
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