Asian parents often tell their children not to sleep with their hair wet. is there actually any scientific reason for this?

No: There is no scientific data about "sleeping with wet hair" that I am aware of. Since hair is not alive, the inside of one's body doesn't communicate with the hair that is beyond the follicle (the body doesn't know if someone is a long-haired person with wet hair, or a short-haired person wearing a wet shirt). However, the wet pillows, sheets, and mattress might be more prone to bacteria and fungi.