Background upload of media is unreliable

### Steps to reproduce

1. Select one or more media files to upload on any room, then lock the phone or close the Element app before upload in complete.
2. It is my understanding that upload should continue in the background
3. only some media files are uploaded, with no reproducible pattern. Most of the uploads just freeze and never complete

### Outcome

#### What did you expect?
That all selected medias get uploaded eventually

#### What happened instead?
No media or only some of the selected media actually get uploaded

P.S. Synapse has MSC2246 turned on

### Your phone model

iPhone 12

### Operating system version

iOS 16

### Application version

Element 1.9.9

### Homeserver

Synapse 1.70.1

### Will you send logs?

No