Unable to Filter a Custom Field with Blended Data

[FC96]

Hello I have a data blend of two tables, with a cross join, i then created a custom field : 

CASE
WHEN CONTAINS_TEXT(description,Hashtag) = True OR
CONTAINS_TEXT(description,Hashtag_two) = True
then description
ELSE 'null'
END. 

Essentially one table has data on facebook posts, the other table is a sheet with a campaign name and hashtags which will be found in the description of the facebook posts. I wanted to be able to select  campaigns and then have the data filtered by the appropriate hashtag of that campaign. The issue im having is i am unable to remove 'null' from table filters as i do not see the custom field i made , nor when i add a control , drop down and recreate the same field, even if i see the right values, when i select one nothing changes on my table.