Exact same problem with mine. Started few days after I started to use it. Happening at least once a day... thats a real problem. Does samsung plan to do anything with the issue ?
For anyone reading this that has this problem I found a potential solution. If you turn off the twilight mode the fingerprint reader works normally again. The problem is that the twilight program puts a filter over the OS and recently its not been passing through the fingerprint scanner. The way to test this is to go to the notifications press twilight and press pause.
No responses from employees means they clearly just made another money maker. Sure enough a "better" phone will be out soon. I have been with Android since the G1 and with galaxy since the s1 n now i have the s8 plus and after only a few days my fingerprint stopped working as well. I suggest switch over to apple as i will be doing. Good luck with Android!