Heading numbering in master document

I’m working on LibreOffice

I cannot change the numbering style of headers in a master document.
All the headers in my subdocuments are numbered in the Arabic style (1.1; 1.2; 1.3 …).
However, the master document that I created from these subdocuments always starts with Roman numbers (I.1; I.2; …).

The paragraph style is the same as in my subdocuments.
The field ‘Format’ → ‘Bullets and Numbering …’ is greyed out and not available.

Where is the fault? How can I change the style of the heading numberings in the master document into Arabic numbers?

Kind regards,

Numbering of headings should be done in Tools - Outline Numbering and not by numbering styles.

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