Only the initial chunk of events gets decrypted if events are loaded from the event cache store.

[poljar]

When using the event cache with storage enabled, events are persisted in the database. Since room keys may not always arrive in the same sync as the event—sometimes arriving later—it is expected that m.room.encrypted events will be stored in the event cache.

Upon reloading and inserting these events into the timeline, the initial chunk of events will get decrypted during timeline construction:

matrix-rust-sdk/crates/matrix-sdk-ui/src/timeline/builder.rs

Lines 490 to 495 in 6c9b1ef

	if has_events {
	// The events we're injecting might be encrypted events, but we might
	// have received the room key to decrypt them while nobody was listening to the
	// `m.room_key` event, let's retry now.
	timeline.retry_decryption_for_all_events().await;
	}

However, when additional events are loaded from the cache, no decryption logic is exists. As a result, undecryptable events (UTDs) will not fix themselves unless the application explicitly requests a redecryption.

[bnjbvr]

I think the way to fix this would be:

• make sure that paginations that come from the cache have an Cache event origin. (I think that's not the case, and they're marked as Pagination if coming from network || storage.)
• then in the timeline, when handling vector diffs which do any kind of insertions, for events originated from the cache, spawn a task trying to decrypt them (or send them via a channel, or whatever is done at the moment to signal that some items must be re-decrypted).

Of course, a better fix would be #3872.