KevinH's guess that it is an all-new-and-improved way of totally mangling mobi7 input sounds reasonable.
I wonder if KDP uploads are now being converted when people upload a calibre-converted mobi7... because they are absolutely insistent on using calibre for some totally inexplicable reason.
Amazon probably wants customers to actually get AZW3 if possible.
You may not be able to duplicate this with *just* kindlegen/previewer.