Autosave is only triggered whenever a change in the project has occured during the autosave interval. The documentation is rather sparse with details, here’s a more comprehensive overview.
Freezing Kontakt tracks should do the trick for this kind of test scenario. When it comes to ARA extensions I am not so sure. You might have to get rid of the extensions altogether. You could save the project and then save it again for test purposes (Save As). Now, you could render your Melodyne tracks and delete the tracks with ARA extensions to be on the safe side. I tend to avoid ARA extensions so there might be a faster way to rule out Melodyne. Thinking about it, you might as well check if freezing Melodyne tracks changes anything first before you do anything else.