May or may not be related to Forum Migration, or else (being still a relative newbug) even occasionally happened with the pre-Migration forum version, but... this could
have a bearing to the Search problems (which stop me easily checking for prior mentions, sorry!), and is definitely something odd.
Composite image here, specifically marked up:
Top: Main (top-level) page, section summarising the "The Comic" sub-threads, with "Individual XKCD Comic Threads" noted as having a "Last Post" relating to comic 1583, Eternal Density having posted at a suitably recent timestamp.
Middle:"Individual XKCD Comic Threads" thread-page, indicating that the latest (unstickied) post was
by Eternal Density, and at that time, but was in comic 1666's thread.
Bottom: After re-ordering, for ease of searching, 1583's thread is indeed found to be last edited back in last October, nothing like announced on the front page.
(Did not bother to check each and every top-level thread-summary vs. its target, for other mismatches.)
...maybe interesting, probably reproducible, possibly investigateable (at leisure). Hopefully not
precipitative towards disabling the front-page summary, for the duration, though...