Having played on my GTW save file more today, I have discovered that the mannequins do not just change outfits when customers try on outfits, but that all mannequins on all lots (including my retail store and the premade retail stores in GTW) have identical outfits. For example, at JF&S Clothiers there are 4 female mannequins wearing the same female everyday outfit and two male mannequins wearing the same male everyday outfit.
Furthermore, whenever I travel to my retail shop, my two mannequins revert to having the same outfit, no matter how many times I manually dress the mannequins.
I'm not sure if I should change the title of this post, as I don't think it completely accurately describes the bug anymore.
I took screenshots, which I will attach below.
Also, I forgot to say in my original post that I play on a Mac that runs OSX Yosemite.
JF&S Clothiers: 3 female and 2 male mannequins wearing the same outfits
JF&S Clothiers: another female mannequin wearing the same outfit
My two mannequins before I travel
My two mannequins after I travel back to my store from another lot
Thanks again!