That happens relatively frequently to me -- but it is only after Touch ID fails to recognize my fingerprint with several tries, usually after washing my hands. I'm looking forward to a Face ID option in the future.
If it isn't due to a failure to recognize the fingerprint, is it happening every 48 hours (i.e., works fine for 48 hours and then a password is required)? That is normal. See the following from Use Touch ID on Mac - Apple Support
For security, you need to enter your password when you start your Mac. Sometimes you need to enter your password to continue using Touch ID. For example, users must re-enter their password every 48 hours and after five incorrect fingerprint attempts.