This is probably related to the upgrade from AES-128 to AES-256 encryption. All clients need to be upgraded to decrypt the AES-256-encrypted notes.
See:
- Discourse post: Android and iOS clients are unable to decrypt notes
- Discourse post: How does the AES-256 update work?
- GitHub issue: Android clients unable to decrypt notes
To clarify, are the clients with the upgrade failing to decrypt notes? (If so, upgrading these instances of Joplin should fix the issue).
I agree that this is very annoying. Ideally, the ability to decrypt AES-256-encrypted notes would have been added several versions before the automatic switch to AES-256.