Using "Expect Unique" assert type on ADF flow with additional streams

Hello,

I'm trying to use ADF functionality assert with an additional stream but it seems it doesn't work.

It recognizes the column name and I can select it but when I try a data preview, I get a "Resolved attribute(s) [...] missing from (list of attributes coming from main source) in operator !Project (list of attributes from main source +  that attribute)"

The attribute does exist in my source, I can see it in the data preview, and use it in lookup. I tried other attributes with same result.

The documentation is not very clear on whether or not this scenario is possible. I'm guessing not but would like confirmation.

Thank you.