While we have special handling for the horizontal displacement of tabs, it seems most system fonts actually do not have a tab glyph itself, leading blender to place the missing-tab glyph in their place. I've evaluated numerous fonts, and searched my entire system font folder for those with tab glyphs, and almost none actually have them. This leads me to believe that I believe blender needs special handling so it does not display a missing glyph for tabs in Text Objects.

The attached shot is just FreeSerif, but the same holds true for almost every other font I try.

Bfont works fine. It is missing the glyph, but 9 refers to U+fffd / .notdef:

Bfont's 9 (with the .notdef ref):

FreeSerif, at U+fffd (I don't see it specifying .notdef, but it is set as a replacement char):

FreeSerif's tab (9)

DejaVuSans.ttf (9)

DejaVuSans.ttf (U+fffd)

DejaVuSans in Blender with tabs