Regression: Creator doesn't notice changes to files

I open my old session containing QtCreator-master project. When I change some source file and save it and do: Build for Run Configuration "qtcreator", it doesn't rebuild the project. Compile Output says:

14:13:34: Running steps for project QtCreator...
14:13:34: Starting: "/usr/bin/cmake" --build /home/jarek/dev/creator-master-build-515 --target qtcreator
ninja: no work to do.
14:13:34: The process "/usr/bin/cmake" exited normally.
14:13:34: Elapsed time: 00:00.

When I open the same session with Creator 4.15 it works fine.