Custom Creatures Stats Not Appearing Until Restart

Hi there! I’m pretty new to the modding community but I seem to have observed an issue with a few mods and tutorials I’ve looked at. When making custom creatures that are able to breed and be tamed, some of them don’t show their actual stats until a server restart while some mods do. Does anyone know the cause of this? Is it part of the settings in regards to breeding or is it how you change a custom creatures stats when making it?

Can you point out what tutorials/guides you were looking at that resulted in this problem?