Ahh I think this is a feature created by SmithingPlus for some reason, I feel silly for missing this. I believe the mod I use causes all iron blooms to have enough voxels when first generated, but the iron bloom I worked on had lost some voxels from an accidental splitting. Since the mod disabled helve hammer regenerating voxels, I couldn't add new ones through it. I will remove this mod so that I can use the vanilla functionality of the helve hammer. Apologies!