When building multiple branches with the same project, but e.g. different Qt versions or modules, they sometimes interfere by linking wrong Qt libraries and using different module includes.
The reason for this seems to be the temporary qmake directory that has been moved from IntDir (which is per checkout) to the system temp (which is global) in 80a0b5e.
This directory and its contents are created for the first build of the project, and then sometimes used by subsequent CI-builds (I assume if one is started while the other is still running), or if you click 'Build' instead of 'Rebuild' in VS.

I've reverted this commit for our CI server, which solved this issue.