I would think that this mutation would start being noticeable at 3 months, as that's when the cub's teeth start coming in. It would be non-lethal, but I think that maybe since the lions would eat less efficiently, they would need more food (e.g. a food that normally refills 50% only refills 25%, because the food spills out of their mouths).

I think it would be heritable, maybe a 10% chance to pass down?

Some potential flavor text:
"[Lion's Name] is spewing chunks of food everywhere while they dig into a meal. The cubs are happy to clean up their scraps."

"[Lion's Name] gives you a toothy smile with a small blep between their bottom fangs. Cute!"