The small caps feature does not work for the Libertinus Serif font (https://github.com/alerque/libertinus) in Illustrator. Only some of the letters get capitalized. It does, however, work with Libertinus Serif Display. (different variant of the same font family) This issue (from the font development page) contains more information: https://github.com/alerque/libertinus/issues/482
Other programs like LibreOffice don't have this issue and render the small caps correctly, so it may be that there is an issue in Illustrator.

To reproduce: just download and install the Libertinus font, create a new document, add some text, apply the Libertinus Serif font and activate small caps in the Character menu of Illustrator.

AI: 25.2.1
OS: Windows 10
Font: Libertinus 7.040