Search by Subject (Currently beta ver. only) require some refining; if the user perform the action on a message reply or forward the entire subject line is searched which return no result given that the search include the prefixes that mail servers automatically tag onto replied and forwarded messages such as: "Re:" or "Fwd".
Please see attached pic as an example and refer to the search bar to see what I am trying to point out.
Currently a workaround is going to the search bar to delete the prefix. The filter then gets applied properly 100% of the times.
Additionally, the filter doesn't work in the main Hub view or if it does then it doesn't filter the calls log out. For the filter to work the user need to select the account in which the message belong to which is too many steps given that often times the user simply manage messages using the main Hub view.