Great diagnosis!

In fact, it was this small change that broke all node sets that were user lists and should reflect the "recent" users:

Originally, there was this query, which did not use an index on lasttime:

                SELECT node_id    pick_id,
                       title      pickname,
                       node_id    auth_id,
                       title      authname,
                       experience rep
                  FROM node,
                       user
                 WHERE node_id = user_id
                  AND (UNIX_TIMESTAMP(NOW()) - UNIX_TIMESTAMP(lasttime
+) < ?)
              ORDER BY experience $args{order},
                       node_id ASC
                LIMIT  $limit
[download]

I eliminated the function on lasttime by converting the date math from math on integers/unix timestamps to math on datetime types:

                SELECT node_id    pick_id,
                       title      pickname,
                       node_id    auth_id,
                       title      authname,
                       experience rep
                  FROM node,
                       user
                 WHERE node_id = user_id
                   AND lasttime > date_sub(NOW(), INTERVAL ? DAY)
              ORDER BY experience $args{order},
                       node_id ASC
                LIMIT  $limit
[download]

which now used days where it should have used seconds:

                SELECT node_id    pick_id,
                       title      pickname,
                       node_id    auth_id,
                       title      authname,
                       experience rep
                  FROM node,
                       user
                 WHERE node_id = user_id
                   AND lasttime > date_sub(NOW(), INTERVAL ? SECOND)
              ORDER BY experience $args{order},
                       node_id ASC
                LIMIT  $limit
[download]

As my excuse, there was a similar-yet-different code path further above, where the same parameters were used for non-user nodes that were in days, instead of seconds :-)).