Fist element in the NCX ToC must not be collapsible, it's a known bug introduced to PW1 with firmware 5.4.x, or maybe it was on 5.3.9 and still it's not fixed on newer PW1 & PW2 firmware. So the fix is to add (via Calibre's ToC Editor, or Sigil) something like "Title Page", ie any single level element before that collapsible element.
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