Messages are suddenly unable to be decrypted

Sometimes, while in the middle of a 1-to-1 conversation, the messages on one of the person's phone will spontaneously be replaced with a bunch of "Unable to decrypt" errors. This applies to previous messages as well as newly sent messages. While this is happening, the messages all show up fine on the other person's phone.

In other words, person A can see the messages from person B, but person B cannot see the messages from person A.

See the screenshot for an example. This was sent to me by my friend during an actual conversation. On my end, everything looked fine and I could see his messages, but he couldn't see mine. His username was replaced with "test_user" for privacy reasons.

- All parties are using the Element iOS client
- Device: iPhone 8, and Later
- OS: iOS 14.1
- App version 1.0.16

Server is a self hosted Synapse instance with a pretty much default config; using SQLite.

If this occurs again, I will submit bug reports from the app from both ends and reference this issue.