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Seems that the pgxn user does not have sufficient database permission to actually select the appropriate rows. I think this is because the contrib schema isn't in the search path, so it's doing a case-sensitive comparison, though I couldn't get it to work no matter what casing I used for the distribution name. So I need to figure out why it didn't work (I worked around it by temporarily using the superuser to connect to the database).
Fixed by granting the "pgxn" user usage of the contrib schema. There's a line that does this in sql/01-roles.sql, so I'm not sure why the user didn't have it in production, but now it does.