Adding a widget caused loss of some previous messages until the widget was removed

### Steps to reproduce

1. Have a chat history
2. Add an EtherPad widget
3. Observe disapperance of messages under specific circumstances
4. Delete widget
5. Observe reappearance of messages

### What did you expect?
For the widget to work as expected.

### What happened?

1. I run a Matrix Synapse server. I was talking with someone on @matrix.org in a private chat, and added an EtherPad widget to demonstrate that Element supported much more than just chat.
2. Following adding the EtherPad widget, the messages disappeared for him, he was using Element on Android. However, they didn't disappear for me on Element Desktop on Windows.
3. On a whim, the timing coincided with adding the widget, so decided to remove the widget for 'everyone' (him and I), which brought the messages back instantly.

Further interestingly, the widget did not work for the user. They were presented with this error when attempting to view it instead of the buttons:


### Your phone model

Unsure of the user's model

### Operating system version

Android

### Application version and app store

Unsure of the user's app version

### Homeserver

matrix.splitanatom.com

### Have you submitted a rageshake?

No