It took me a while until I figured out why the ligatures were broken in the default theme.
There is a CSS default in the base theme that breaks ligatures for headings it’s [letter-spacing], I’d propose to remove it from the default theme.
Tested with Cascadia Code on windows.
A workaround is custom CSS with this.
* {
font-family: "Cascadia Code", monospace !important;
font-variant-ligatures: normal;
font-feature-settings: "liga" on;
text-rendering: optimizeLegibility;
letter-spacing: normal !important; /* FIX LIGATURES IN HEADINGS */
}