See this edit: https://musicbrainz.org/edit/61884917

According to the editor:

Not sure why the merge is showing the same release as the target and source. The source is actually https://musicbrainz.org/release/fd94be5b-a636-4fb3-bf9a-cfdccc81ac8f and the target is https://musicbrainz.org/release/8198e778-5021-42df-a7fd-2ac982d16171. The source has no medium, so the merge strategy is correct.

It looks like showing the target in the source list is normal for this type of merge. But it seems that the source with no media is not shown at all in the edit interface, making it very confusing to voters (me in this case!)