stopped getting new events from room, can't rejoin it

Nobody has claimed this yet.

*This issue was originally created by [**@zephryn**](https://github.com/zephryn) at .*

### Background information
- **Dendrite version or git SHA**: still on 0.3.8
- **Monolith or Polylith?**: monolith
- **SQLite3 or Postgres?**: sqlite3
- **Running in Docker?**: nope
- **`go version`**: 1.15.8 linux/amd64

### Description
a few days ago i noticed that a room i was in wasn't receiving new events.

when trying to leave the room, i get the following error:
```
Error leaving room
User "(matrix account)" is not a member of room "(id of room i can't join)"
```
attempting to rejoin the room returns a different error:
```
InputRoomEvents auth failed: eventauth: eventauth: "(matrix account)" is not allowed to change their membership from "leave" to "join"
```
this message is similar to the one shown in issue #1023.

trying to rejoin also produces these logs server-side:
```
level=error msg="helpers.CheckAuthEvents failed for event, rejecting event" func="processRoomEvent\n\t" file=" [github.com/matrix-org/dendrite/roomserver/internal/input/input_events.go:106]" auth_event_ids="[(event id)]" error="eventauth: \"(matrix account)\" is not allowed to change their membership from \"leave\" to \"join\"" event_id="(event id)"
level=info msg="Error authing soft-failed event" func="processRoomEvent\n\t" file=" [github.com/matrix-org/dendrite/roomserver/internal/input/input_events.go:120]" error="eventauth: \"(matrix account)\" is not allowed to change their membership from \"leave\" to \"join\"" event_id="(event id)" room="(id of room i can't join)" type=m.room.member
http: TLS handshake error from (client-side ip goes here): EOF
```

Start by reading roomserver/internal/input/input_events.go around lines 106 and 120, then compare the reported leave-to-join auth failure with issue #1023. Reproduce the failed leave and rejoin flow using the reported monolith, SQLite3, and Dendrite 0.3.8 setup if possible; done means the room receives new events and the user can leave or rejoin without these errors.

Written by the indexing model from the issue text.