The page name strategy is not applied when performing an Office import with document split

Steps to reproduce

• In section "Administration > Editing > Name strategies", select and save the "Preformatted names" strategy
• In LibreOffice, create a document with several level 1 headings containing spaces, such as "Heading ABC" and "Heading DEF", then save it in ODT format
• Create a page using type "Page from Office"
• In the "Office Importer" screen, select the ODT file you just created, check "Split document" along level 1 headings, and hit "Import"

Expected result

The document sections are splitted into subpages named "Heading-ABC" and "Heading-DEF" (spaces got replaced by dashes).

Actual result

The resulting pages are named "Heading ABC" and "Heading DEF"