As of recent Riot/web versions old messages that can't be decrypted in rooms will not be shown (saves the user from being blasted with Unable to Decrypt errors)

As of recent Riot/web versions old messages that can't be decrypted in rooms will not be shown (saves the user from being blasted with Unable to Decrypt errors). So the messages are likely there, but you won't see them until you import your e2e keys.