TL;DR
WordPress 6.9 introduced a fix for adjacent post navigation when posts have identical publication dates. While this resolved a long-standing bug, it inadvertently caused infinite loops in some extensions that modify the get_adjacent_post() WHERE clause.
What Changed in WordPress 6.9
In WordPress 6.9 (Trac #8107), a bug fix landed where next/previous post navigation failed when multiple posts shared identical post_date values. This commonly occurred when bulk-publishing draft posts.
The Technical Change
The get_adjacent_post() function’s WHERE clause was modified to include ID-based comparison as a tiebreaker:
Before (WordPress 6.8 and earlier):
WHERE p.post_date > '2024-01-01 12:00:00' AND p.post_type =