It may very well happen that rows are left unmasked not because of an error in the algorithm but because of logic. In such a case, the logs will show all rows masked because all rows have been processed, however based on logic they may not have actually masked.
In short, if you are looking for a confirmation of data actually is masked or not, I am not sure logs are the right source to give you that information.
If you are taking a backup of your tables before masking, then may be its just easier to join the backup table with the table masked on the primary key and look for before and after values that are same and not null- ideally if everything is masked you should have 0 rows from the join