Scheduled posts published spontaneously

I’m running several wordpress blogs on a server. For several years all have been fine. I did not change or install anything recently. Each blog runs its own database.

The problem is as follows:

two blogs have a lot of scheduled posts. Simultaneously, both blogs started publishing scheduled posts on wrong dates.

For example, on Dec 19 both blogs published 4 posts each: Dec 21, Dec 28, Jan 04, Jan 28. Erroneous posts are published along with one legitimate scheduled post.

All erroneous posts are published with correct (future) dates. The server and blog date is correct. Thus, on Dec 19 my blog proudly showed posts dated Dec 21 to Jan 28.

There is no apparent system in publishing the erroneous posts. Several scheduled posts between them were not published. That seems to rule off spontaneous time changes on the server (if server date would be somehow changed from Dec 19 to Jan 28, all posts in between would have been published rather than a few).

Both blogs publish erroneous posts for the same dates.

It seems that “Published” status is randomly assigned to scheduled posts – but only to those scheduled for the near term.

Any ideas are appreciated.