Super- Sub-script glyphs generation

A mystery about superior and inferior.
I created the superior and inferior glyphs from a to z. Now, if I compile something with Xe- or Lua-LaTeX like

\superscript{abc}

everything is fine.
But if I write instead

\superscript{łñẑ}

that is using glyphs I present as lowercase but that I did not create even in the superscript version, in the text they do not appear.

But if I do the same thing with some other fonts that I've tried (EB Garamond, Garamond Premier Pro etc.), even if I didn't find those glyphs in the sups), they also appear in the superscript version.

Obviously I am missing something in the generation of these glyphs ...