[Arkanosis]

My (imperfect) understanding is that this is because of how end-to-end encryption works: you not only need to receive the messages (which are stored on the server, so you don't have to worry about them as you can retrieve them when you want), but also the keys to decrypt these messages (which are only stored on the clients, so whether or not they are available depends on you).

Possibly, one of your clients has the keys needed to decrypt one of the messages, but you're using another client which doesn't. Things go back to normal when both are connected at the same time and can share the keys, or when the client of the sender is connected and still has the keys.

If you don't keep your clients connected all the time, you can use a secure backup on the server, so the clients can retrieve the encrypted keys from the server and decrypt them locally.

Not having the keys happens more often if one the parties uses short-lived sessions (like logging exclusively in a private browser window, for example).

This article helped me with understanding a little: https://gerstner.it/2021/02/matrix-and-e2e-encryption-or-how...