Chunk slimes don't spawn slimes often, I've done several distance tests and nerfed the world just for spawn slimes, but nothing solved, even in the hard way. I noticed that if you change the peaceful mode back to the hard mode they will appear again, but only for a while, and back you have to repeat this method several times.
I hope you solve this, thanks.
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Thank you for your report!
We're actually already tracking this issue at MCPE-21856, so I will resolve and link this ticket as a duplicate.
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