Just gave my new A42 some cheap screen protector and faced that issue as well. Came a bit as a surprise to me. Anyway, I covered some 20-25 % of the screen with a slightly moisture, soft and absorbing tissue (in my case about 3 mm thick). I coved the other end of the screen, not too close to the sensor. With a little bit of trying the sensor did then read the fingerprint. To my surprise, it works afterwards just as normal, same as before without the screen protector. Just play around with this idea and it might work for you as well.
I found somewhere on internet that the screen protector messes with the fingerprint reader’s ultrasonic tech. So, I thought it might get too many interference signals, since the screen is, in a way, now thicker and the sensor is not calibrated for it. Hence, ‘catching’ those signal waves and preventing them from going back to the sensor could be a solution. And now it works because the sensor reads and processes the signals coming back from the finger(print) before the interference signals arrive (travelling across all the screen and back).
Well, just a theory, and yes, I might be totally off here. Please, feel free to tell me this is complete nonsense, but an explanation why it works would be nice. If it is just a sensor calibration issue, one cold come up with a fingerprint sensor calibration app (if there isn’t one already).
