I'm a bit reluctant to run a overall repair as it may corrupt other Projects.
You still have your backup, don't you? So you could revert to your saved state, if need be. After you set up a new user on a nrew machine I'd at least epair the permissions with the first aid tools.
How did you restore the Aperture library from the backup? Was it done with as part of a setup with Migration Assistant? Or did you restore the library separately from Time Machine?
Does the project in question have managed originals or referenced originals? You may need to reconnect referenced originals after migration.
You cannot repair just a single project, but if there is an inconsistency between Browser and Viewer, regenerate the thumbnails for the images in the project, also the previews.
Select the images in the browser and use the command "Photos > Generate Thumbnail". When that is done, hold down the ⌥-key and use the command "Photos > Generate Preview".
Are your thumbnails now consistent with the Viewer image?
Strikes me as odd that the Browser browser is not showing the same thing as the Split View browser.
Perhaps there is a difference in Quick Preview mode, Kirby? I have no time to experiment right now.
It would really help to know, how JD8001 migrated the user account and Aperture library, and if the user account name is still the same, to see, if there is a settiings problem or the library needs repairing or rebuilding.
-- Léonie