RFC-4013 SASLPrep is a prerequisite for implementation of SCRAM-SHA-256. We currently only fully support SASLPrep on  the .NET Framework version of the driver. 

Relevant snippet from the ApiDocs:

In .NET Standard, authenticating via SCRAM-SHA-256 may not work with non-ASCII passwords because SaslPrep is not fully implemented due to the lack of a string normalization function in .NET Standard 1.5. Normalizing the password into Unicode Normalization Form KC beforehand MAY help. SCRAM-SHA-1 is the recommended alternative for now.