Unable to Mask data after applying mask function in Azure Synapse table
It seems that the user has full rights to the database or at least she/he can get unmasked data.

Make sure that this user is not one of the 'SQL users excluded from masking'.

Quoted from docs:

Dynamic data masking permissions
Dynamic data masking can be configured by the Azure SQL Database admin, server admin, or SQL Security Manager roles.

Dynamic data masking policy


SQL users excluded from masking - A set of SQL users or Azure AD identities that get unmasked data in the SQL query results. Users
with administrator privileges are always excluded from masking, and
see the original data without any mask.
Masking rules - A set of rules that define the designated fields to be masked and the masking function that is used. The designated
fields can be defined using a database schema name, table name, and
column name.
Masking functions - A set of methods that control the exposure of data for different scenarios.