Hi there!

Unfortunately I have some bad news for you. There is nothing we can do for the missing content. World Anvil is not meant to be used in multiple devices or even tabs concurrently. This is clearly stated in multiple places in our documentation. When you fire up an inactive tab in the same or a different device, the browser loads it from its cache. When this older state is loaded the autosave system detects the change in content and triggers a save, thus overwriting content you may have put in in another tab.

Be aware that this in not something specific to notebook, but applies to all of World Anvil. There are plans for a system that mitigates this behavior but it is still some time away. Until that system is ready for release it is best to avoid working in multiple tabs. Always open a new tab when switching devices.