[DropDownList] Sticky group headers don't work in Fluent theme

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Description

The sticky headers are empty when using the Fluent 12 theme.
Curiously, this doesn't appear to be a plain CSS issue. The ".k-list-group-stick-header > .k-list-header-text" element does not have any text content.

Steps To Reproduce

https://codesandbox.io/p/sandbox/4lh9j5

Screenshots or video

Actual Behavior

Empty sticky header

Expected Behavior

Sticky header with text content of the current group section.

Browser

Chrome, All

Browser version

latest

OS type

MacOS, Windows

OS version

11

Last working version of the Kendo UI for Angular package (if regression).

No response