GloriousRez The harem menu is a fucking plague on my fucking existence.
Okay, I think that what has to have happened is as follows:
You did NOT get Wendis' new body until Chapter 5, after Sabitha joined the harem, but you DID get Wendis' body AFTER you downloaded the new Steam update, and you got Sabitha joining prior to that.
Up until the latest update, Sabitha joining put Wendis's portrait in the harem menu, while Wendis joining added Sabitha's. In the scenario I describe above, you got Sabitha, adding Wendis's portrait, got the update, and then got Wendis' body back, adding Wendis again.
Does that sound right?
Every time you talk to Yarra to check the harem list, it sets the variable with the number of harem members to the number of visible portraits, and the achievement fires if you are at 36. I'm somewhat surprised that you got the last harem member while not rebodying Wendis at the earliest opportunity, but your assumption of how many there are may be wrong.
So it appears the code is working right, you just got screwed by the fix.
Editing this requires setting a script function rather than any of the variable and switches accessible via the save editor, and getting you access to a dev console is so complicated it's easier to just say:
Go back to before you go to the Lustlord's Tower for Ginasta and replay to the end. If, (as I suspect), you don't have the last harem member anyway, you'll almost certainly need a replay to get the achievement.
Out of curiosity... what does the white text on the red hat of your forum avatar say?
It's quite illegible.