Membership event list summary hides leave events with no prior join when hidden events are enabled

### Steps to reproduce

This one's a bit niche:

1. Enable 'Show Hidden Events' under labs
2. Crack open devtools in a room you're admin of
3. Send an `m.room.member` state event for a user ***not*** in the room, eg `@alice:localhost`, with membership `leave`
4. Do the same, but for a second user (`@bob:localhost`)

### What happened?


Only the first leave event is shown. It's clear that the second one is being trapped by MELS due to the lack of timestamp on the membership event (meaning the timeline code knows there's an event which follows, but it assumes that the grouping would take care of it). However, it did not get grouped.

### What did you expect?

For the second (hidden) membership event to be either part of a MELS or otherwise shown in the timeline, as it used to.

### Operating system

Windows 10

### Browser information

Edge 92.0.902.67 (Official build) (64-bit)

### URL for webapp

local build

### Homeserver

localhost (Synapse 1.40.0)

### Have you submitted a rageshake?

No