Expected
The project should be saved, with a progress bar indicating the involved save step
Actual
The Save-As dialog is invoked instead, with the default save-as filename example.fcp.fcp
It seems to me that the FC creates the <save filename> by <filename>.<extname>, rather than the <basename>.<extname>. Hence the save-as dialog appears.
Once the project is saved by FC16, this issue can not be reproduced. I guess there is compat issue behind the scene here.
FontCreator 15.0 cannot open font projects (*.fcp files) that have been created or saved from FontCreator 16.0. That is why you see the Save As dialog. The old project file is saved in the new project file format.
FontCreator 16 can save its own project files without showing the save as dialog.
Sorry for the confusion here, the issue here is not opening FC 16 projects by FC 15, but rather the opposite: Open FC 15 projects by FC 16, which is normal when upgrading to a new version.
Aside: Currently it is impossible for FC 15 macOS to open a project saved by FC 16 macOS, as the program will be stuck at the start scene.
Here is the updated reproduction steps, I hope it is more clarified.
Reproduction
Use FC 15 to create an empty project, save as example.fcp