Scheduled Posts not updating lastBuildDate in feed

[tempspace]

For the past few months, I've been running into an issue where scheduled posts are not updating the lastBuildDate field in the XML. The post itself gets put into feed just fine, it's just the single field that doesn't get updated. Unfortunately, MailChimp uses this field to detect if anything new has happened, so from its side, there are no changes.

To update the lastBuildDate, I have to edit the newly published post, and then click Update (without making changes). Once I do that lastBuildDate is updated. If you diff the XML from before and after, that field updating is literally the only change.