It looks like not all editions with a publisher get merged.
See screenshots for the two entities I was merging, and compare to the result merge:
https://bookbrainz.org/publisher/9ccc8de5-f6ed-4eaf-9e3a-bfd0ba1c4243

The editions of e6e35692-94f9-48f0-bb76-f51caa46d8b2 aren't there anymore, and trying to visit any of them (e.g. https://bookbrainz.org/edition/7071d7ed-62d6-43bd-9eaf-3e36b483a605) I get a timed out request.