i believe fissures are due to iron deficiency and vitamin a deficiency, and not so much directly related to diarrhea, but it is related to diarrhea for sure.
Iron is known to boost retinol binding proteins which enables retinol(vitamin a) to be delivered to tissues. vitamin a is very important regulate of epithelial(outermost skin) integrity, and this includes the intestine as well as the more well known skin on our body.
so it is indirectly related to chronic diarrhea. in crohns disease,disease mainly affects the ileum and this is where most if not all fat soluble vitamins are absorbed(a,d,e,k). With so much mucus being produced, the fat cannot be absorbed and therefore the vitamins cannot be absorbed either and escapes digestion and will manifest itself as floating fecal matter, because oil/fat floats upon the surface of water. if you arent absorbing your dietary fat, you aren't absorbing your fat soluble vitamins.
so it is most likely the fat soluble vitamin a, and second most likely iron intake that is involved in fissures, where the skin becomes thin, dry and loses its elasticity and then cracks and has a hard time healing, the anal orifice requires great amount of elasticity to function properly.