We've been able to trace the issue to what I believe to be some aggressive caching options on certain Wordpress installs. Calls to update_post_meta and get_post_meta are always returning a cached value. I can understand the get_post_meta, but update_post_meta is a volatile action and shouldn't be cached here.

To get around this, I can run delete_post_meta, but then the original WP_Query is cached still so we keep processing the same posts over and over again (needlessly). The only way I've been able to get it to work properly, consistently is running wp_flush_cache(). However, this seems quite aggressive from my perspective.

I also tried wp_cache_delete to target the individual items, but since I'm not setting them into the cache, I can't reliably delete them from the cache.

Side note… one specific Wordpress installation does seem to be using a pretty old version of the Redis cache.

Original conversation happening in Slack.