Low spawn rate of Slime in single player

Put the summary of the bug you're having here

*What I expected to happen was: There would A a large amount of slime always spawning because in every single player I had I always have huge mines that go on for like 18 chunks. at least one of them have to be slime chunks!

*What actually happened was I only get one huge slime every 20 minutes or so when i go down to my mine.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
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