FIXED: If you are using Optifine, disable Lazy Chunk Loading in Video Settings >> Performance
I have the same problem, the world was created in 1.12. I tried recreating it in version 1.14.3 but slimes still do not spawn on the same chunk. I used Chunkbase's Slime chunk finder and Amidst to find these slime chunks.
Don't see a solution posted.
FIXED: If you are using Optifine, disable Lazy Chunk Loading in Video Settings >> Performance