Form + basic authentication cause problem

I have setup site in IIS 7.5 and .net 4. Because its QA site I have set basic authentication on. While DNN (Site) require From authentication. This was used to work well till some .net framework updates has broken this setup.

Previously Request.IsAuthenticated was "false" unless SetAuth Cookie is called. But now as soon as user perform initial basic/windows auth - Request.IsAuthenticated is "true". This make app to think user is already logged in.

Is there any way to fix this problem?