I had the same problem, had a fully cycled tank according to testing, added shrimp, had to remove one of the filters bc the shrimp wouldn't stop getting stuck in it & bam nitrite spike out of nowhere off the charts and I could not get it to go down no matter what.
It is finally down to the 1 and below range now. It took a week and a half of daily 25% water changes using fritz detoxifier as my dechlorinator as well as adding full doses of it to the tank daily. I'm now doing 50% water changes and adding the full dose of fritz detoxifier and adding seachem stability to rebuild my bacterial colony. I think it would have gone down faster had I been doing 50% water changes and adding full doses of the detoxifier/bacteria from the get go, I was worried the large water changes could stress my shrimp to death but the fear was unfounded in my case. Before I started doing all this I was losing 1 cherry shrimp a day, after I started doing this I did not lose any more shrimp and hopefully will not lose any more!! (Knock on wood lol)