With VS 16.3.2 and QTVSADDIN 1.4.1 Intellisense is unable to find include files when importing a QMake (pro) file. I'm currently using several projects connected with a SUBDIRS pro file. The solutioin will still build successfully, but all the includes and references to their contents are marked as undefined in the code windows.
I found I could import the pro file, close the solution and select yes to save the solution, then reload the solution directly to fix the problem. Setting properties on each project in the solution with a small change to the additional include files will also fix the problem, but takes much longer.