Other issue about shrimp is the hardness of the water. A diet properly containing minerals and healthy foods with help form a good shell. When there is too hard water, not enough acidity then the hardness of the shell can be too hard and the shrimp might not be able to shimmy out of the shell and be stuck in it and die, with an incomplete molt.
Did yours die partially molted?
Also after re-doing a tank, many times, if filter pad or substrate is changed then there can be a mini cycle?
What parameters did you test for when you said the water was fine? And do you know your TDS
total dissolved solids, pH? By any chance, GH
If it is a strip test it may not be accurate. I lost nearly a whole tank of fish before even after monthly double checking the test strips with a liquid test, which was great until the time when it was wrong.