I'm surprised 0.18.4 has more instances than 0.18.5, considering 0.18.5 was the simplest update ever with just a small hotfix for federating moderation actions (which is a really important bugfix) and it didn't have any changes to the database or configuration or dependencies
Having a look, the only instance with more than 250 monthly active users on 18.4 is Beehaw. They decided to not update after their disagreement with the Lemmy devs.