This is due to a limitation in Bear app, which was also reported here: Rich-link in Bear by shortcuts broken - #4 by LucB
Until they provide an x-callback-url API for getting the URL of the current note, Hookmark needs to rely on UI scripting, which in this case is unreliable. You should be able to get the title using the normal Copy Link keyboard shortcut ( ⌘C), or Copy Markdown Link ( ⇧⌘C). (There are also universal variants of these commands).