This is actually a very welcome Project setting, so now we don't have to change the Scratch Disk settings every time we open a different project. (I've always got about half a dozen going at any given time, and changing the global Scratch settings every time I opened one was a major hassle. This new way is MUCH better.)
Thanks - think our situations are a bit different. I have multiple users on the single system so reminding each to set the scratch drive each time they create a project will be a mission.
We used to have Premiere Elements which had a single scratch location, within which each project had its own subfolder. Hence my expectation that Pro would operate in the same way.
Perhaps Adobe could provide us with the option of working both ways - a global setting which can be overridden within the project.
Create an empty project with the Scratch settings you want, and have each user always start with that (doing a Save As... before any actual work is done.)