Create forum as a link placing it on top in the first category, check index.
Last Topic Title on Forum list
To clarify, for reproducing the issue, the link forum must be the first forum listed on index, and not within any categories or forums.
And, here is the error:
[phpBB Debug] PHP Notice: in file [ROOT]/includes/functions_display.php on line 469: Undefined variable: last_post_subject_truncated
A link forum is not the only requirement for this error and is simply an example. A subforum displayed above a forum's topics will also show the error.
Fix looks good.