I have noticed this about my broken and cracked plumeria branches. If I find a branch that has broken but not broken off completely, I tape it up securely with painter's tape. I have used electrical tape but found that the occlusion of air often makes it rot. While the attempt to help the branch heal and grow together only worked once, what usually happens is that what's left connected to the cambium layer keeps the branch green and growing. After a few months and after removing the tape, there are often 3-4 thick aerial-type roots that have developed. I have successfully cut the branch that has calloused on it's wn and instantly potted up the cutting in soil where it grows right away!