You aren't the only one suffering from mysterious deaths.
I'm in the process of re-cycling a tank. Everyone has been telling me that it's the water, the wood, the lack of stability in the tank, too many large water changes, the substrate or the tank itself. (among some other reasons) The tank has never had fish in it, I was having deaths even while using RO water, had deaths even with stable parameters, it's the same type of wood that is being used in the other tanks, there were a max of 4 large water changes done within 7 months, I have the same substrate from the same bag in another tank and I *know* it's not the water. Three tanks that all get the same water regardless of what water I'm using (remineralized tap, remineralized RO, or a mix of hard and soft tap). Only having problems in one tank.
I ended up moving all the shrimp to a temporary tank with "pure" tap water (no RO or remineralizers) treated with Fluval Aqua+ Plus, Mosura ShiZhen Power, Mosura Old Sea Mud Powder and Ebi-Ken Sosei. It has driftwood that has been used in other tanks, indian almond leaves and alder cones in it, all of which have been boiled. Coming up to 3 weeks in the holding tank and have had zero deaths. Previously, it was up to 1-3 deaths a week.
The tank I'm re-cycling (ammonia, nitrites and nitrates) just recently finished going through the ammonia cycle, so I hope to add the shrimp back into the tank soon. Still not entirely sure what has been causing the deaths...