Probably just a missing NBT tag.
https://minecraft.fandom.com/wiki/Slime#Entity_data

This one is interesting. IIRC, slimes aren't actually supported by this mod since slime spawning conditions are so ... conditional. I can't immediately recall, but I thought that spawners can't spawn slimes, or can only spawn them in slime chunks, or similar, of which is too much management for this mod to worry about when spawning dungeons randomly.

Something like
https://feedback.minecraft.net/hc/en-us/community/posts/360068768752-Slime-spawner

I'm surprised you got slimes to spawn via this mod. Can you add any more details?

When I wrote the Issue, I modified the BlazeRoom spawner MobTypes to only SLIME and MAGMA_CUBE.
It is possible that, on that time, I was on a Slime chunk (therefore Slime conditions met), but this time (as you mentioned) Slimes indeed didn't spawn.

For the attached video's test, I'm using a non-modified BlazeRoom spawner.
Magma Cubes always spawn at the smallest size.

I compiled with your fix, I even changed the randomSlimeSize() return value with a 3, but Magma cubes still all spawn with Size = 0 ... 😫

@KameiB Try it out with a value of 127 ;)

Specifically, there's some Forge funkiness inbetween the RLD code and Minecraft, so its important to remember that the interfaces might not match 1:1 with Minecraft's specifications.

Try out your initial experiment again -- I was able to witness large slimes spawning.

However, noting again, this mod does not actually support slimes since the spawners for slimes generally don't work (as this mod does no checking for slime chunks, etc.).

I did this as a labour of love for you! :D

Seemingly accidentally closed