This also happens in Top Menu not populating options.
I think I know what the cause is:
The line is the bottom line of the topic map, and seems to extend if the post below (2nd post/1st reply) is posted after a long time, e.g. ‘4 months later’ or ‘6 years later’. This is the case in the 2 topics I’ve linked, and doesn’t happen in others.
Update:
Allow normal users to bulk update notification level also has the problem, and has the same occurrence of the ‘xxx years later’ (in this case, 4 years).