dynamic data masking read uncommited
Dynamic Data Masking doesn't affect stored pages and it will work the same way with any transaction isolation level.

from Microsoft documentation(https://docs.microsoft.com/en-us/sql/relational-databases/security/dynamic-data-masking?view=sql-server-ver15):


  Dynamic data masking is easy to use with existing applications, since
  masking rules are applied in the query results.