Not possible to add user to board and correct project

[Lena Sjölin]

Hi! 
My first question in this community and I hope someone can give me an explanation why it's working like this. 

We have a project PMO Finance (PF) that has a board that shows issues from two different projects, PF and FSSCPM. 

When we try to add user to that board/project (by pressing the + sign in picture below), the user is added to the other project, not the project where this board is located. 

The board filer looks like this:

Why is user added to FSSCPM project instead of PF? 
Is there anything I can do so it works like we would like it to?

[Susanna Babayan]

can you try to add them via project settings > people ?

[Lena Sjölin]

Yes, adding them from People is working. I just want to understand why it's not working from the board.
For new people in Jira I believe it's easier for them to just press the + sign. And I assume I will get this question again so I thought I try to find out the answer.