It's designed to grab page titles and use that as the link text. If your page titles contain the forum name, the link text will too.
There are some exceptions to this. Where possible we aim to get the title instead from the social network metadata rather than the title tag. In the case of XF threads, that doesn't contain the board title. However most other pages don't have any social media metadata so the page title is used and of that contains the board title then the board title will be used in the link text.
This can't really be selective. It's equivalent of asking if links to Google.co.uk could be titled Google instead of Google UK.
The best workaround might be to have a solution custom developed which specifically removes the board title from the title if it exists or to update your templates to remove that from the title. Otherwise it's working as expected.