In an "application" model rather than a "document" one, like MS Word online or draw.io or similar, there's no clear semantics for "back" but there is a risk of the user losing data if they can navigate away without saving.

This is a consequence of sites being allowed to hijack back in the first place. They can still fix it.

For your use case all you need is the page to get notified so it can save. Remember that on Android your onSaveInstanceState gets called and you have to save your state or lose it.