I have just run a backfill to fix all occurrences of this across all sites on the network (332 instances of it happening on the entire network that needed to be fixed, including 216 on SO). So if this appears for any questions closed after 2020-04-06T14:08Z then it means that the issues is still happening.
Issue with the edit/original screen for mods and gold tag users has also been fixed.
This issue has hopefully been fully addressed, the fix is on all network sites.
The issue itself had to do with cases where either the poster or the user who raised the close flag had low enough rep that they could not yet access the review queue, and thus the system marked their flag as a
RecommendedClose. This led to weird scenarios in which the closing mechanism would accept the self-close action of the poster (following the recommendations for duplicate posts given in the post notice), but the system failed to record the actual
OriginalDuplicateQuestionId value for the post (leading to the empty list).
Thanks to Adam Lear as well for help with this. We are pretty sure that we have identified the root issues here, and have added unit tests as well to verify the fix. I have also added logging here that will record more details in any cases where this will happen again moving forward. If you see any (with question closed after this update), please report them in a comment.
There is no easy way to retroactively fix all posts affected by this. However, mods and gold-tag users should be able to use the
Edit function given at the top of the duplicate post notice to add back the missing duplicate question (without having to reopen and close). If you're not a moderator or gold-badge user, or you are a gold-badge user but can't find the right target in the post's comments, flag the post for moderator attention with a clear note about this bug and a link to this post; a moderator will fix it.
Thanks everyone for your patience with this and for your help reporting instances where this occurred.