I took a plane from Hong Kong to Yangon (flight number: Cathay Dragon KA 250; Airbus A321 (twin-jet) according to FlightAware) and I noticed some flashing lights in the plane cabin throughout the flight. Video: https://youtu.be/3ccvmWNxTu0
Picture with and without the light:
Pretty much all segments lighted up at some point during the flight. I couldn't detect any lighting pattern.
Why do some planes have such flashing lights within the plane cabin? It lasted the entire flight.