Gray shield on messages from a verified user's iOS devices in a certain room

### Steps to reproduce

1. A fully verified contact (with green shields on his username and on all of his device sessions in the room members sidebar) sends messages in two encrypted rooms.
2. When that contact sends from his iPhone in Room A, my Element Desktop session 1 displays a gray shield on his message.

The gray shield does not appear in any of these circumstances:

- When messages are sent from my contacts' Element Desktop sessions.
- When messages are sent in Room B.
- When messages are viewed in my Element Desktop session 2.
- When messages are viewed in my Element Android session.

This is happening with two senders, both of whom use Element iOS, in the same room. Both of them currently see the following message in their Element iOS client, under Settings > Security > Cross-Signing:
> ⚠️ The MSK is missing. Verify this device again or use the Secure Backup below to synchronise your keys across your devices

(maybe related: vector-im/element-ios#4460)

### Outcome

#### What did you expect?

I expect messages from fully-verified sessions in an encrypted room to display with no shield.

#### What happened instead?

One of my Element desktop instances displays a grey shield next to some of these messages. It seems to happen only in a certain room and only on messages sent from iOS.

### Operating system

Linux

### Application version

1.9.3

### How did you install the app?

Flatpak

### Homeserver

matrix.org

### Will you send logs?

Yes