Stopwords do not get applied to unigrams because Stopwords are used for full text search; they are applied at the N-gram stage. In order for the Stopwords to be applied, change the N-gram length equal to 1.
To modify the N-gram length in your workspace, go to Settings and click on the model you want to modify. At the bottom of the pop-up, click “Switch to advanced setup.” Then, go to the parameters tab at the top. Here is where you can modify the length of the N-gram.