此问题正式为调查中"(已报告并回答:

This issue is officially "Under Investigation" (reported and answered: https://developercommunity.visualstudio.com/content/problem/160939/bug-adding-xaml-content-page-to-shared-project-xam.html) and there is a workaround:

一种解决方法是安装通用Windows平台开发工作负载

a workaround is to install the Universal Windows Platform development workload