different output for the same file

[Paulo Ney de Souza]

Would anyone shed some light why the MWE:

----
\documentclass{standalone}

\begin{document}
Świa̧tek
\end{document}
----

would produce different outputs between pdfLaTeX, xeLaTeX and luaLaTeX?

pdflatex: It does not process and outputs an error.
xelatex: Process silently and replace the a-ogonek by an "a".
lualatex: Process silently and replace the a-ogonek by an "a"+ space.

Paulo Ney
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