In CMake projects, PROPERTIES DEBUG_POSTFIX is ignored

We have a workflow to configure cmake on console first, because a lot of variables have to be set.
We would then "open" the existing debug and release build folders in qtcreator. To that end, we first open the CMakeLists.txt and then adjust the build directory paths ("Details") to point to the existing debug resp. release folders. That workflow worked well but since some weeks ago the executables are sometimes not found (no way to find out, which tool was changed in the meantime, sorry)
In CMakeLists, a DEBUG_POSTFIX is specified (that used to work in the past)

Reproduction

We could only reproduce it with "CodeBlocks - Ninja" generator

• Extract the attached minimalistic CMake project
• configure it on the console using the build.sh script
• Open Qt Creator
• open File/Project - select CMakeLists.txt
• do NOT use the automatic Kits, use a Kit that has the same cmake selected, as was called by the build.sh (e.g. default version in Ubuntu 16.04)
• select the existing debug und release folders
  
• Configure project ... wait for cmake. The Debug config is selected initially
• click Run-> Creator attempts to start "demo" executable, not "demo.debug" and fails
• switch to Release and Run -> works
• switch back to Debug and Run -> now also works can be used as a workaround

Ninja: 1.5.1,
CMake: default version in ubuntu 16.04, also fails with newer cmake.