Be careful when adding any new live plants with shrimps in there. I lost all of my Japonicas a few hours after adding some new plants. They literally died one after the other - As I was netting the first one out, another died. Then another was obviously not well, but before I could get him, another died. And so on. Within ten minutes of the first one dying, all 6 were dead. Some aquatic plant growers treat their plants with a copper-based snail killer. Any traces on the plants can be fatal to shrimps.
Ive also heard that copper test kits dont necessarily pick up small traces of copper. So even if your test shows 0, there may still be traces that would be enough to harm your shrimps.
If you have a bundle of them huddled together in the corner, there is definitely something wrong. Especially if they huddle around the filter pipe or try to get out of the water. Could well be the pH swing though.
If solving the pH swing doesnt solve the problem, try looking at other things. Do you use liquid plant food? A lot of them have traces of copper in. What else do you put in the tank? Have you used any meds lately? Have you added any decor that has come from another tank that may have been treated with something?
Hope you solve it soon.