DIAGNOSIS The touch panel's light button is unresponsive, or a recent firmware/app update has a bug that ignores the setting.
FIX Force a reboot of the control system. Unplug the purifier from the electrical outlet for a full 2 minutes. This fully discharges any residual power. Plug it back in, turn it on, and immediately try pressing the physical Light button on the unit to cycle the light off.
IF THAT DIDN'T WORK Check the app's schedule or automation rules. You may have an old 'Scene' or 'Automation' set in the VeSync app that is overriding setting and turning the light on at specific times. Delete any automations related to the light.
HEADS UP The light has two brightness levels. A quick press toggles it on/off. If you press and hold the Light button, you are adjusting its brightness, not turning it off. Make sure you're doing a quick, firm tap.