The issue is traceable to the way isequal handles objects.  The function uses

s = builtin ("struct", obj);

to convert an object to a struct.  For classdef objects this returns a struct where all properties (including read-only, etc.) are fieldnames along with their value.

However, for old-style class objects this seems to return a confused struct object which, when indexed, reverts back to a class object.

octave:2> a = audiorecorder ();
octave:3> typeinfo (a)
ans = class
octave:4> class (a)
ans = audiorecorder
octave:5> s = builtin ("struct", a)
s =

  scalar structure containing the fields:

    recorder = 0

octave:7> typeinfo (s)
ans = scalar struct
octave:8> class (s)
ans = struct
octave:9> y = s.recorder
error: struct: additional arguments must occur as "field", VALUE pairs
error: called from
    __get_properties__ at line 36 column 13
    disp at line 35 column 3
y = audiorecorder object with properties:

octave:10> typeinfo (y)
ans = audiorecorder
octave:11> class (y)
ans = audiorecorder

I think the issue needs to be resolved in struct().  Given that the call

obj = class (STRUCT, "classname")

creates an old-style class object, it should be relatively easy to revert back to a struct.

Adding jwe to the CC list because this is plain-old weird.