options_for_select selected value not working

The documentation states:

(...) 
options_for_select(container, selected = nil) public
(...)
(million examples)

So you shouldn't pass selected: something, but just something. Also, this something needs to be selected value, not text:

= f.select :user_id, options_for_select(@users.pluck(:name, :id), @projectuser.id), include_blank: true