The ultrasonic in-display fingerprint sensor on the Galaxy S10 was the first of its kind when the Galaxy S10 series was launched earlier this year, but it had one limitation: It didn't work well with tempered glass screen protectors. The ultrasonic fingerprint reader works by firing sound waves at the user's finger and reading how those waves are reflected back, and standard tempered glass protectors create a slight gap between the finger and the display that reduce the sensor's effectiveness.
Accessory makers overcame this limitation with liquid-based tempered screen protectors, which use liquid adhesive to form a tighter bond between the protector and the phone display. However, this turned out to be a major security flaw for one Galaxy S10 owner, who bought a ‘gel screen protector' off eBay and found that any fingerprint could unlock the phone once the protector was installed. Samsung has said that it is investigating the matter and has simply blamed the issue on a cheap screen protector for now — the one on the affected device costed around $3.
Ultrasonic in-display fingerprint sensors may be the best solution for front-facing fingerprint readers now that smartphones have edge-to-edge displays that leave no space for physical buttons, but they aren't perfect. Their incompatibility with tempered screen protectors is unfortunate, and it seems only plastic ones allow ultrasonic fingerprint readers to work without issues. That might be why Samsung has said since day one that Galaxy S10 owners should stick to official screen protectors from the company, none of which include a tempered glass option.