Preserve snapshots when merging documents

I used to be able to merge documents and the list of snapshots would merge as well. Now that no longer sems to be the case. Has something changed? What is the expected behaviour when merging documents?

I read on an old thread on the forum that it might be a display issue, so I tried it again today and lost the snapshots. I have tried clicking away, re starting Scrivener and a few other things to try and prompt it to refresh and show the full list but no luck.

Losing half the snapshots when merging really makes the merge feature unusable. But, I have other merged documents with combined lists of snapshots from a while ago (June last year), so I know this used to be possible. Do I need to do something differently?