Alica, I didn’t realize openntpd just ignores the leap second. Those probably aren’t appropriate for the pool then, at least not in regions of the world with plenty servers.
That being said, it’s not super clear what to do to improve the situation. Exclude servers if they fail a leap second? It’s somewhat random depending on which upstream servers they have at the moment and lots can change until the next leap second (might be in 6 months, might be in 3 years).