Using CONFIG += debug_and_release, sub-directories "release" and "debug" are created and used for building.
The resulting application binary is (by default) placed in the main directory.
Hence, when switching between both builds (e.g. "make debug", do some testing, then "make release"), the app binary ALWAYS needs to be replaced with the new one.
But that does not happen if the new target has already been built before and didn't change (and hence no new linking is done).