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.