From my understanding, GH and TDS "kind of" go hand in hand... i.e. the higher the GH, the higher the TDS. However, it is possible to alter GH without changing TDS? (still learning about GH, KH and TDS)
I was thinking that perhaps the difference between the two is too great for the shrimp to handle. Even if you say the water from all sources is "about the same", it doesn't account for the different total dissolved solids... Does your LFS dose ferts in their tanks? If they are, are they dosing the same as you are? Or less? More? Although the wild shrimp do live in water where there is a lot of decaying material and the nutrients and minerals from the soil add to the water, if they are in a stream, that constant "fresh water" running through can keep those things down as compared to living in a pool that may not be receiving as much "fresh water".