When displaying the feed title for each entry, consider the case in which the feed is not in the scope yet.

This can happen if the user clicks on a folder to read its entries; there may be entries returned by the server which have just been fetched and that belong to feeds that have not yet been sent to the client (they will be sent in the next automated client refresh; in the last such refresh they had no unread entries, but now they do).
OPML files exported from TinyTinyRSS are correctly imported.

It seems TTRSS does not give folder nodes a "title" attribute, only a "text" attribute. Also it includes some <outline> nodes directly under the <body> which are not folders but TTRSS-specific configuration options, which we should ignore.