Tabs are stripped even with the preserveTabs=true Lua option enabled

[Witiko]

Steps to reproduce bug

Create a text document named test.tex with the following content:

\input markdown
\input lmfonts
\catcode`\ =13
\catcode`\^^I=13
\def {\ (SPACE)\ }
\def^^I{\ (TAB)\ }
\input{optionfalse}

\input{optiontrue}
\bye

Using a text editor, create a text document named content.md with the following content. Replace the graphical tab characters (↹) with actual tab characters:

These words are separated by spaces and these↹are↹separated↹by↹tabs.

Afterwards, execute LuaTeX:

$ texlua `kpsewhich markdown-cli.lua` -- content.md optionfalse.tex
$ texlua `kpsewhich markdown-cli.lua` preserveTabs=true -- content.md optiontrue.tex
$ luatex document.tex

Expected behavior

Executing LuaTeX should produce a PDF document named test.pdf with the following content:

These (SPACE) words (SPACE) are (SPACE) separated (SPACE) by (SPACE) spaces (SPACE) and
(SPACE) these (SPACE) are (SPACE) separated (SPACE) by (SPACE) tabs.

These (SPACE) words (SPACE) are (SPACE) separated (SPACE) by (SPACE) spaces (SPACE) and
(SPACE) these (TAB) are (TAB) separated (TAB) by (TAB) tabs.

Actual behavior

Executing LuaTeX produces a PDF document named test.pdf with the following content:

These (SPACE) words (SPACE) are (SPACE) separated (SPACE) by (SPACE) spaces (SPACE) and
(SPACE) these (SPACE) are (SPACE) separated (SPACE) by (SPACE) tabs.

These (SPACE) words (SPACE) are (SPACE) separated (SPACE) by (SPACE) spaces (SPACE) and
(SPACE) these (SPACE) are (SPACE) separated (SPACE) by (SPACE) tabs.