"Some of your messages have not been sent" when replying to a threaded message that uses io.element.thread rel type

[frakic]

Steps to reproduce

1. Where are you starting? What can you see?
   -I start a thread within a public room with a client which uses io.element.thread prefix (e.g. iOS)
   -I switch to a client which uses m.thread (develop.element.io) and I see the message in my timeline, as expected
2. What do you click?
   -Using develop.element.io, I try to make a threaded reply to this message

Outcome

What did you expect?

Not reconciliating the two threads is fine but the user should at least be able to send the message.

What happened instead?

The message could not be sent at all.

Operating system

Windows 10 Home 21H1

Browser information

Firefox 98.0 (64-bit)

URL for webapp

https://develop.element.io

Application version

Element version: f149f9a-react-1eb67047c90d-js-9fc8048c307f Olm version: 3.2.8

Homeserver

matrix.org

Will you send logs?

Yes

[t3chguy]

2022-03-11T14:51:42.279Z I sendEvent of type m.room.message in !VBDnsRtbNNtchetwWf:matrix.org with txnId m1647010302279.1
2022-03-11T14:51:42.428Z E Error sending event MatrixError@https://develop.element.io/bundles/16963f9153af95a415d9/init.js:54808:5
parseErrorResponse@https://develop.element.io/bundles/16963f9153af95a415d9/init.js:54747:13
requestCallback/<@https://develop.element.io/bundles/16963f9153af95a415d9/init.js:54697:17
doRequest/req<@https://develop.element.io/bundles/16963f9153af95a415d9/init.js:54621:18
on_end@https://develop.element.io/bundles/16963f9153af95a415d9/vendors~init.js:209427:13
on_state_change@https://develop.element.io/bundles/16963f9153af95a415d9/vendors~init.js:209377:7
instrumentXHR/</</</<@https://develop.element.io/bundles/16963f9153af95a415d9/vendors~init.js:191566:41

[t3chguy]

This is an error from the server

Cannot start threads from an event with a relation