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"Not used for over a week" is the whole answer. Safari clears script-writable storage, localStorage and IndexedDB both, after seven days without user interaction with the site. It is a privacy feature and it will keep eating your mornings.
The documented exemption is sites added to the home screen. Get the iPads to install it: valid manifest with display: standalone, then Share and Add to Home Screen. Installed web apps keep their storage.
While you are there, move off localStorage anyway. It is synchronous and capped around 5 MB, and a half-written JSON blob on a backgrounded tab is its own failure mode. IndexedDB with a small wrapper is the right store for a queue of pending records, and you get transactions.