SQL Security :: Data Masking Removed After Joining With Other Tables In CTP 2.1?

I find that dynamic masking does not work on joining tables in SQL Server 2016 CTP2.1.

For examples, I create the following table:

CREATE TABLE [dbo].[HRM_StaffAppointment](
[StaffID] [nvarchar](11) NOT NULL,
[ApptSeqNo] [smallint] NOT NULL,
[ReportingDept] [nvarchar](10) NULL,

[Code] ...

Then I apply mask on StaffID and RankDesc.

alter [dbo].[HRM_StaffAppointment] alter column [StaffID] add masked with (function='default()')
alter [dbo].[HRM_StaffAppointment] alter column [RankDesc]
add masked with (function='default()')

When User A logged in and query on HRM_StaffAppointment, StaffID and RankDesc are perfectly masked. But User A can remove the masking using another table:

CREATE TABLE [dbo].[staffID](
[staffID] [nvarchar](255) 
) ON [PRIMARY]
select a.*
from dbo.HRM_StaffAppointment as a
left join dbo.staffID as b
on a.StaffID = b.StaffID

It looks like a security hole to me, or I'm doing anything wrong?