My Galaxy S7 is locking me out when it can't read my fingerprint instead of asking me for my pin.

[Android Central Question]

I had my Galaxy S7 set up to ask for my fingerprint and if it doesn't recognize my fingerprint it normally asks for my pin. But twice now instead of asking for my pin it has locked me out of my phone for increasing amounts of time. So the first time it's like 20 seconds and if it still doesn't recognize my fingerprint it locks me out for a minute, the 5 minutes, then 10 minutes, then 30 minutes and then an hour. So I was unable to use my phone for almost two hours. Fortunately after the hour long lock-out it recognized my thumbprint and let me back in. Because I'm concerned with being permanently locked out of my phone I've removed the thumbprint security option. What is happening? Why does it stop giving me the option of entering my pin when it doesn't recognize my thumbprint?