I'm curious.... I don't see certain info about your setup, so please forgive me if I've overlooked it.
How old is the tank?
What are the water parameters, and what devices/ test kits are you using? By this, I mean what is: ph, ammonia, nitrite, nitrate, specific gravity, etc. and what brands of kits are you using (API, Elos, Salifert, etc.).
My experiences indicate heavy metal toxicity, poor water quality, and induced sources are the main culprits. All of which have been brought up.
If you could include actual numbers, rather than "all parameters are ok", it would help a ton!
Have you had any hand lotions, or colognes on by chance that may have entered the aquarium on your hands..? ANYTHING wierd that could have gotten in the tank via aerosols?
Also, I didn't notice a protein skimmer listed for your tank. How are you accomplishing nutrient export? Water changes, refugium, etc??? Could the nitrates/ nitrite be slowly climbing, and have become toxic to the shrimps?
Please let me/ us know.
Would love to try and help prevent any further losses.