Thank you for the quick reply. Yes it was still visible in Audition's File/Open Recent menu but when I tried to open it from there this morning I began seeing a message saying ~after_crash.sesx doesn't exist, would I like to search for it?. I searched everywhere on my machine and in the backup folder for it, looked at every .sesx in both of those locations, and eventually opened all .sesx files ever saved from the project in all folders to see if any had the new activity. None do (and now they all show up instead of the _after_crash.sesx in my recent sessions listed in Audition.)
My search for the .sesx is feeling exhaustive at this point. It looks to me like when I recovered that backup a few days ago, the Adobe backup system considered the newly named and saved after_crash session as an old backup to be deleted. If so I guess it's because it was in the backup folder and/or still linked somehow to project.sesx despite having been renamed and then saved many times. I worked on it so much in the last couple of days that it is possible I'd never closed it, and/or had never reopened project.sesx, and doing one of these two actions today triggered the deletion.
I would so like to be wrong about this so I definitely welcome further troubleshooting.
I am happy to say I found the ~after_crash.sesx by going to Adobe Cloud's desktop app and searching the deleted files in the file system online. Phew!