messages sent while offline cannot be decrypted

### Steps to reproduce

1. being offline, send several messages to an existing contact (to which you didn’t send a message in a little while, in my case, if it matters)
2. go back online, and wait for the messages to be sent

### Outcome

#### what did you expect?

that the recipient could decrypt the messages

#### what happened instead?

the message could not be decrypted by the recipient. sending extra messages while being online did not solve the problem. i eventually used `/discardsession` and messages sent after that were able to be decrypted, but not the previous ones.

note that this happened with existing devices (sessions) that sent many messages successfully to each other for a long time. no new sessions are involved in this.

it seems to me that element created a new cryptographic session, but because it was offline, it could not ask the server for the device (session) list of the other user. having something like matrix-org/matrix-doc#3354 would probably help with this. 😉

this happened between users on separate homeservers: synapse 1.44.0 sending to synapse 1.33.2.

### Your phone model

_No response_

### Operating system version

android 9

### Application version and app store

element 1.3.12 from google play

### Homeserver

_No response_

### Will you send logs?

No