Wow, you did a good acclimation job, no doubt. i always go for a longer than normal time too.
I would say another fish or invert got it but you have nothing in the tank that I think would go for a cleaner shrimp. I had 5!! yes 5, die on me, but I think it's something in my tank getting them.
Maybe someone will jump in, I'm at a loss. I mean sometimes you get a bad fish or invert and no matter what you do they are destined for dying. Kinda sucks but it happens.