I managed to track the problem to the forum search include, where $fieldsvar (which is used in both queries, effectively turning them into monsters) yields this:
(thread_subject LIKE '% %' || post_message LIKE '% %')
This is supposed to include my search words, but it doesn't. As far as I can tell, the following two lines of code are the issues:
I don't know why they're there, but they've been around since 8.00.19 on Github, and removing them fixes the issue. They reset the $swords-array which is used in the search_querylike()-function, rendering the latter a (useless) monster in the forum and every other big table.