Why did notes disappear from my trash?

Trash is not an archive: by default, anything in it is permanently deleted after 30 days. The switch sits in the Trash view itself, labeled "Auto-delete after 30 days" - turn it off there if you want trash kept forever, and the setting syncs to all your devices. The cleanup runs on your device when the app opens, because the server cannot see which encrypted notes are trashed.

Permanently deleted means gone: no server copy and no recovery path. If you use trash as a someday-maybe pile, disable auto-delete or restore notes before day 30. Until then, trashed notes still count toward your storage - "Empty" in the same view frees that space immediately.