Even I am facing issues with fingerprint scanner with tempered glass. But I face that issue WITHOUT tempered glass also. It works after 2-3 attempts.
This ultrasonic fingerprint scanner is crap. My earlier mobile's (21FE, still with me) fingerprint scanner works flawlessly in comparison.
Edit: From Samsung forum and it works much better now.
"The trick is to go into the fingerprint settings, and use check added fingerprints function. Scan your fingerprint starting from the center and each time move your finger more and more off center, with each successful scan it will actually learn the edges of your fingerprint, and it will be much more reliable.
My fingerprint scans 90% reliably after doing this."