Batch Change preview loses all changesets after a few days

• Sourcegraph version: 3.26.3
• Platform information: Chrome 89 on Mac OS

Steps to reproduce:

1. Create a Batch Change preview from a batch spec
2. Verify that the preview link shows the Batch Change with all of the expected changesets
3. Come back after a few days

Expected behavior:

I'm not sure if this is a bug or just an aggressive expiry of changesets created during a preview since previews are more ephemeral. If it is just an aggressive expiry, I would expect preview changesets to persist for much longer than just a few days. And I would also expect a message explaining that the changesets were discarded because the preview was created "N" days ago or something similar, rather than just an empty preview.

Actual behavior

The same preview link now shows a Batch Change with no changesets, like so:

I verified that this was not a mistake with the batch change spec by re-running the exact same preview command via our CI workflow, which created a new preview with all of the expected changesets that I had seen originally.