Project not up-to-date (always rebuilding)

I noticed that some Qt Vs Tools projects are constantly rebuilding without any valid change. I turned on the "Detailed" build output and found the common line for each of these projects':

Project is not up-to-date: build input '{...}\Debug\qmake\qtvars_{...}.{...}.designtime.props' is missing.

I used {...} to denote variable text that can differ from project to project.

In each of the cases, I do not see the mentioned file anywhere in the project definition (.vcxproj) file. It also only happens in Qt VS Tools project - thus I assume that the issue is plugin-related.

Please investigate. This issue has increased our re-build times from under a minute to close to 10 minutes.