Cannot send messages to some recipients

### Steps to reproduce

When trying to send messages to signal recipients I see some kind of preflight thumbnail fetch which fails and then end up back on "Messages failed to send".

### Outcome

The server says

```
2022-05-23 22:54:32,734 - synapse.access.http.8009 - 427 - INFO - GET-7021- CLIENT_IP - 8009 - {None} Processed request: 0.014sec/0.005sec (0.006sec, 0.000sec) (0.003sec/0.004sec/2) 93B 404 "GET /_matrix/media/r0/thumbnail/bitcoin.ninja/IMAGE_ID?width=90&height=90&method=crop HTTP/1.1" "Riot/20220505163017 CFNetwork/1333.0.4 Darwin/21.5.0" [0 dbevts]
2022-05-23 22:54:32,748 - synapse.rest.media.v1.thumbnail_resource - 382 - INFO - GET-7022- Failed to find any generated thumbnails
```

Note that the image ID is the recipient's profile pic, which is a jpeg and loads just fine if I download the image itself (image is 1024x1024 natively).

### Your phone model

_No response_

### Operating system version

iOS 15.5

### Application version

Element 1.8.14

### Homeserver

synapse 1.57.1

### Will you send logs?

Yes

Start by reproducing the failed send on iOS 15.5 with Element 1.8.14 and inspect the preflight request for the recipient avatar. Compare the thumbnail 404 and “Failed to find any generated thumbnails” log with the successful original image request; done means messages send successfully for recipients whose profile images trigger this path.

Written by the indexing model from the issue text.