First, the kitten is too young and hasn't fully developed its eyelids yet. Just like some children have one single eyelid and one double eyelid, which gradually develop into double eyelids over time, it takes time. The owner can continue to observe for a while to see if the eyeliner on the kitten's other eye has formed.
Secondly, although most cats are born with natural eyeliner, a small percentage of cats are born without eyeliner or with only one eyeliner. This may be related to the cat's genetics and generally does not have any adverse effects on the cat's health, so there is no need to worry too much.
Finally, if a cat's eyes, which normally have a liner-like appearance, suddenly become partially invisible, it's very likely that the cat has an eye infection or inflammation. This is usually related to factors such as the cat being overheated, bacterial infection, or viral infection. If you notice that your cat's eyes are starting to become red and swollen, or that there is increased discharge, it is recommended to take it to a veterinary hospital for examination as soon as possible.
