Experience shows that webxdc can be the source of quite some space leakage.
As a workaround, you can create a new profile where you keep all your important content. You can then disable automatic deletion of messages from the device on that profile, and either
- Keep disappearing messages disabled in chats where you use such important apps
- Disable disappearing messages right before sending in an important app and then restore it to its default value of 5 weeks after sending. Apps are scoped to the lifetime of the original sent message that contained it as an attachment.
This could be made even more friendly if it was possible to override the timer per message or per message kind (i.e., automatically disable it for apps):