Thanks for the update. I'll have to go give a sniff to one of my older soy wax soaps, Dawni. But I do use a different soy wax than you (due to differing local availability, I know). Perhaps the rancidity may be related, or maybe the grapeseed oil?
Since all else was equal, including simultaneous stirring (!), perhaps the preservatives in the lard vs none in the soy OR the 2% unknown additives in the soy had something to do with the DOS in one and not the other soap. Did you add any preservatives? ROE? BHT? Commercially sold lard generally has BHT added to it (at least here in the US), so that soap may be less prone to rancidity than the other soap if no additional preservatives were added.
More as it relates to the lard/soy comparison and my personal experience:
Lard soap has a very smooth marble-like surface and my soy wax soaps do not. None of them are as hard as the lard soap, either. I wonder how your recipes compare for the feel of hardness as you hold them/use them.
In my experience, soy wax does not compare well to lard, neither for hardness nor for the smooth marble-like surface. Of course our recipes differ, so it may not be your experience. I really believe the overall recipe influences the soap more than any single ingredient. Still, we do like to know what each ingredient contributes, don't we? I know I do.
And the olfactory factor, which does vary for some folks as not all noses are equal:
A few days ago I did a hand-washing test with a soap I thought was a palm oil tester soap (that I had made around the same time I had made the lard soap), so it was a blind test in terms of I didn't know what the ingredients were when I picked it up and started washing my hands. I had to go check my records when I shockingly thought to myself, "This smells awful; it smells like lard." Yep, it was the lard soap I made 4 years ago. The photos are in my records to proove it, along with the recipe; not the palm oil soap at all. Now, some folks are fortunate enough not to have the sniffer of a bloodhound, but not me, I am gifted with a sniffer that can smell a variety of things not everyone can sense, including lard in cured soap, and I gotta say, sometimes it comes in real handy and sometimes I hate it. So for those who think the lard smell goes away, it really doesn't, not for some of us. That's okay, I do have non-vegetarian friends and some family not gifted with my sniffer who are happy to get lardy soap. It is hard as a rock though, and has absolutely no DOS.
So, I am off to sniff out an older soy wax bar soap of comparable age.
ETA: I don't have any of comparable age as I didn't start using soy wax that long ago. I'll have to look at newer soaps - in the 2 y.o. age range, next.