You need to manually reset the indicator. It doesn't auto-detect a new filter.
With the purifier on, press and hold the Baby Sleep Mode button (the one with the moon icon) for 3 seconds. The Check Filter Indicator will flash three times and then turn off.
If that didn't work, make sure you're holding the correct button for a full 3 seconds. If the light comes back on immediately, the unit may have a fault. The indicator is timer-based, factoring in usage. If you run the purifier 24/7, it will come on sooner than 12 months.