It's not InnoDB specific bug.
Error is in QUEUE code, about the way we handle queue->max_at_top.
It's either '0' or '-2' and we do '^' operation to get the proper
direction. Though queue->compare() function can return '-2' as
a result of comparison sometimes. So we'll get
queue->compare() ^ queue->max_at_top == 0 (when max_at_top is -2)
and _downheap() function code will go wrong way here:
...
    if (next_index < elements &&
        (queue->compare(queue->first_cmp_arg,
                        queue->root[next_index]+offset_to_key,
                        queue->root[next_index+1]+offset_to_key) ^
         queue->max_at_top) > 0)
      next_index++;
...

Fixed by changing max_at_top to be either 1 or -1, doing
'* max_at_top' to get proper direction.


include/queues.h:
  Bug #31890 Partitions: ORDER BY DESC in InnoDB not working
  
  max_at_top policy changed
mysql-test/r/partition.result:
  Bug #31890 Partitions: ORDER BY DESC in InnoDB not working
  
  test result
mysql-test/t/partition.test:
  Bug #31890 Partitions: ORDER BY DESC in InnoDB not working
  
  test case
mysys/queues.c:
  Bug #31890 Partitions: ORDER BY DESC in InnoDB not working.
  
  queue->max_at_top policy changed - now it can either be '1' or '-1'.
  We multiply comparison result on max_at_top to get the proper
  direction.