Diagnosis
The filter reminder is on a timer, not a sensor. It activates every 40 cycles. It needs a manual reset.
How to Fix It
With the dryer off, press and hold the Temp. button and the Signal button simultaneously for 3 seconds. The 'Check Filter' light should turn off. That's it.
If That Didn't Work
The lint filter housing itself has a small magnet and a sensor. If the filter isn't seated perfectly, the sensor won't register it. Remove the filter and re-insert it firmly until it clicks.
Heads Up
This light is just a reminder. It doesn't mean the filter is actually full. But always clean the lint filter after every single load for safety and performance.