The new Samsung Galaxy S10 phone has a faulty fingerprint sensor that approves unregistered fingerprints (meaning a thief can unlock your phone, even if they don’t have your prints). A software update will be released to fix the problem soon.
The problem is exacerbated or caused by screen protectors, and users have reported that 3D printed fingers can unlock it as well. However, it’s best if you don’t rely on the fingerprint feature (using a password or something else in the mean time) until Samsung fixes it.
Regardless of the phone (whether or not it has this glitch), I would use passwords to lock any financial or email apps to be safe. Fingerprint scanners are not invincible (and I can’t really say they’re reliable either).