Missing xmpp federated messages

**Describe the bug**
This room has replies to user messages that are not being bridged.

**To Reproduce**
1. Join https://matrix.to/#/#xmpp_onfoss_conference.hribhrib.at:matrix.org
1. History starts at 2021-04-11
1. Today at 15:11 UTC [is clearly a reply](https://matrix.to/#/!wZHmKuwbDAgPrepPeC:matrix.org/$AVXmvs1Mj0IEY-RoIagDlVj2ENq_GRS-df-WF4LjpOQ?via=matrix.org&via=freedombox.emorrp1.name&via=t2bot.io) to a missing message
1. Neither ThinkSome, nor L29Ah show in the User list, and I have reason to believe they're joining via federated XMPP

**Expected behavior**
1. View the entire conversation
1. See all current users in the User list

**Screenshots**

**Desktop (please complete the following information):**
- OS: Debian 10
- Browser: Electron (assumed, Element Flatpak)
- Version: 1.7.24

The issue names no source files, tests, or entry points. Start by reproducing the missing reply and absent users in the linked XMPP room, then trace the bridge behavior for federated XMPP messages. Done means the full conversation is visible and current federated users appear in the user list.

Written by the indexing model from the issue text.