Heading
Headings get style “myHeading”, “mySubheading” and “mySubsubheading”. They were set to Outline Level “Level 1”, “Level 2” and “Level 3” respectively in the properties dialog of the style.

Subheading
None of this styles has assigned a numbering style. The UI in LibreOffice does not allow to not set the style:list-style-name attribute, but always set style:list-style-name to empty string. This disables numbering in LibreOffice. I have removed the style:list-style-name attribute manually.

Subheading
TOC is set up to use “Outline”.

Sub-Subheading
Dummy text

Sub-Subheading
Dummy Text

Heading
Dummy Test