Push notification content display is extremely unreliable

**Describe the bug**

Despite enabling both settings related to push notifications in Element settings, actual pushes almost never include the message content (with rare exceptions). E2E rooms are more problematic than plain-text. But in the end, given sufficient time, no pushes will include content at all.

Sometimes there will be a room name in the push message, but most of the time not even that.

**To Reproduce**

Steps to reproduce the behavior:
1. custom homeserver
2. login, enable notifications
3. join rooms (plaintext and e2e), make sure they are unmuted
4. switch out of the application
5. wait some time
6. send messages in the rooms

**Expected behavior**

Push notifications that arrive on the device include message content.

**Screenshots**
(TBD, filing from desktop)

**Smartphone (please complete the following information):**
- Device: iPhone X, iPad
- OS: iOS 13.1, iOS 13.3
- Version: `1.3.3 TF #20210416`

**Additional context**

Some pushes _will_ have message content included.
As more time passes since hiding the app, the probability of receiving a push with content decreases.