The algorithm switching is probably based on whether a full decomposition is requested (3 output argument) or just the singular values themselves (1 output argument).  Both svds (z) and svds (z,2) sort the singular values in descending order when there is only one output.

Confirmed.  Changing Item Group to "Matlab Compatibility".

Additional information, svds() can return just the singular values, rather than the full decomposition.  When only one output is requested, Octave does return a list sorted in descending order.

Secondly, not all complex matrices are sorted in ascending order.  The example below (generated with random integers), works.

x = [
   35   19   41   29
   25    4   30   50
   55   49    5   37
   59   47   22    9
];

y = [
   63   53    1   33
   13   38   24   17
   20   54   32   57
   25   17   60   59
];
z = complex (x,y);

[U,S,V] = svds (z)