I must say...it is very weird that the way you are doing it is not working. I tried it on my end and could not get it to work either.
To get it to work, I wrote something like this:
SELECT "Date"."Date" FROM "Sales-SubjectArea" where "Date"."Date"=current_date
The key here was that I added a 'where' clause and it seemed to work.
So maybe yours could be something like:
Select "DIM_PERIOD"."DATE" from "<subject area>" where "DIM_PERIOD"."DATE" = TIMESTAMPADD(SQL_TSI_WEEK,-2,Current_Date)
(I realize that the query I give may not really make sense in your subject area -- like you may not actually have 'Date' in your DIM_PERIOD dimension. But I'm just trying to show how maybe using the WHERE clause might help)
Hopefully this helps!