I think what is happening is that they are trying to make it really realistic or just ended up like that without actually realizing it
IRL slag sometimes can trap some iron due to how this type of smelting works, i think that its the same thing happening here
You smack some slag off and it might take off 1 voxel of iron with it, get unlucky and you end up with voxels missing due to how the game works
If anything we are lucky that slag doesnt generate holes in our blooms
IRL blooms were just random sized chunks with sometimes slag trapped inside and took long time to work them out of the metal
While its annoying at least helve hammer can fix them and you should have mechanical power at that point of progression
In my opinion the only thing they should do about this is to add some information in the handbook, that if you cant finish the bloom you need to use helve hammer
Ill be honest it still irks me that 1 bar is 1 knife blade instead of at least 2 but this is not the topic on hand