1. Create & configure a project.
2. Open a file of the project.
3. Activate "Restore last session on startup" in the session manager.
4. Close Creator.
5. Delete the generated *.pro.user file.
6. Start Creator again. Because of the deleted *.pro.user you are put into the Projects mode to configure the project.
7. Switch to Edit-Mode - the file is only syntax highlighted.

This happens only for the last visible file/editor in the session. When switching to other files, it's all fine. Also for newly opened files.