Ah the way Prismic handles preview tokens expiring is not great apparently. It just expires but then serves you stale data even though it claims you're no longer previewing. At least this is how it is with sveltekit and preview on localhost. Removing the expired cookie fixed things, but really when your preview is no longer working.. shouldn't it just.. pull live data?
The preview cookie is generated in your browser and should always be cleared using the 'X' button of the preview toolbar.
So, the issue you were seeing only existed in your browser and no one else's. If preview mode isn't exited correctly, then your browser cookie will try to grab a ref that no longer exists.
A preview cookie expires when the preview data is updated and/or the data has been published.
If you see this again, a good way to test is to open your site in incognito or in another browser.