I think it's called wet cracking and has to do with the tallow. Getting wet and drying will cause the cracking. There's a US patent for a bar soap having improved resistance to cracking for a 60%-85% tallow bar. Apparently it's a common problem!
Are you open to using lard? 50% tallow, 25% lard, 12.5% CO and OO? I think the cracking also had something to do with the remainder of the test batches containing coconut oil.
Here's a recipe I found. Just a thought! Good luck!
Same here. I make a 65% tallow soap with no cracking issues, but I don't have coconut oil in it, so maybe that's why? It's just tallow, olive oil and castor oil.