Hi,
The following plain luatex document
\tracingonline1
$b_{\hbox{\char 13}}$
\bye
Produces a warning
Missing character: There is no ã (U+00E3) in font cmr10!
in texlive 2015 luatex (0.80)
There is no warning from pdftex or xetex or from the luatex 0.79 in TL 2014.
David
$ luatex t1
This is LuaTeX, Version beta-0.80.0 (TeX Live 2015) (rev 5238)
restricted \write18 enabled.
(./t1.tex
Missing character: There is no ã (U+00E3) in font cmr10!
[1{/usr/local/texlive/2015/texmf-var/fonts/map/pdftex/updmap/pdftex.map}])<</usr
/local/texlive/2015/texmf-dist/fonts/type1/public/amsfonts/cm/cmmi10.pfb><</usr/
local/texlive/2015/texmf-dist/fonts/type1/public/amsfonts/cm/cmr10.pfb>
Output written on t1.pdf (1 page, 17458 bytes).
Transcript written on t1.log.
