The bug

One of the trial spawners in the trial chambers can spawn slimes. It has two variants in size of the slime: 1 and 2. However the slimes that naturally spawn in slime chunks and in swamps are of sizes 0, 1 and 3 (small, medium and large). I would expect the trial spawners to only spawn slimes of sizes 1 and 3 (medium and large).

How to reproduce

1. Find a trial chamber with a slime trial spawner

   /locate structure trial_chambers
   

2. Activate the spawner
3. Wait until you get a slime with a size between medium and large.
4. Observe its Size

   /data get entity @e[type=slime,sort=nearest,limit=1] Size