I was doing my everyday messing-around-with-mods-in-creative routine, and when I when I was looking at Ceramics features, pouring out liquid made the bucket become iron (only works in Survival, had discovered this because I used /gamemode 0 to test a few things with damage from another mod) and thus Creative will not have this issue). I have tested this with only Ceramics and Forge in a new instance, so it not caused by another mod.

TLDR: Clay buckets with liquid become iron when emptied.

Seems Forge changed something in the way its universal bucket works. I fixed this for the next build