I set I:"Hematite Cluster Size"=48
and then mined a hematite pluton and got 260 odd bits of iron, 1 out of each block. Is this the way it's meant to be ?

Other thing: It appears to be spawning a single Magma Block with Limonite and Azurite plutons.

Part 1: I'll look into it at some point (boy, you sure push a lot of issues...)

Part 2: Yes, that's intended. There's a 1% chance of a block in a pluton being a magma block if you're below Y=24. I had it as lava... not sure which I prefer. Magma would geologically makes sense (magma = molten earth beneath the surface) but I'm still debating if it minecraft-ically makes sense.

Consider if Beta-testing :) I'm putting together our first 1.12 mod pack, and this am reporting many issues and/or suggestions to many mod makers :)

I think either is fine - kind of looked like it was something left over from testing :)

Tested out one last time just now; seems this is fixed.