Rokkitt os2 underline values are zero (broken)

**Describe the bug**

Current variable TrueType Rokkitt is invisible in LibreOffice 24.2.7.2 420(Build:2) on Ubuntu for amd64. Metrics work. Legacy fixed Rokkitt OpenType faces render, but underlining is invisible. The problem with the fixed OpenType faces is present in LibreOffice 7.6.7.2 and absent in LibreOffice 6.4.7.2.

**To Reproduce**

Install LibreOffice 24.2.7.2. Install variable Rokkitt (Rokkitt-VariableFont_wght.ttf dated 2025.9.11). Type some text in Rokkitt. Follow with text in another font to demonstrate spacing. Close LibreOffice. Uninstall variable Rokkitt and install fixed Rokkitt (Rokkitt-Regular.otf dated 2020.3.30). Type some text in Rokkitt. Underline it. Export a PDF and note that the underlining is omitted in the PDF.

**Expected behavior**

The user would expect text and underlining to render in each case.