Diagnosis
The sensor for that light isn't actually checking for lint—it's a timer. It illuminates after a set number of cycles to remind you to clean the filter and, more importantly, the exhaust system. The reset is manual and easy to miss.
How to Fix It
With the dryer plugged in and in standby mode (not running), press and hold the "Signal" button on the control panel for about 3 seconds. You should see the "Check Filter" light turn off. That's it. This resets the cycle counter in the control board.
If That Didn't Work
If holding the Signal button doesn't work, try a full control lock/unlock cycle. Press and hold the "Control Lock" button (the padlock icon) for 3-5 seconds until it beeps or the icon changes, then repeat to unlock it. Sometimes this clears stuck reminders. Then try the Signal button again.
Heads Up
Don't ignore the reminder's intent. While the light is just a timer, use it as a cue to also vacuum out the lint trap housing and check your external vent cap for blockage. That's where real fire hazards build up.