MWE:
-----BEGIN cutting here may damage your screen surface-----
\documentclass{article}
\usepackage[T1]{fontenc}

\begin{document}

\makeatletter%
\fontfamily{Roboto-TLF}\fontseries{c}\fontsize{12pt}{12pt}\selectfont%
This is \f@series{} and should be c.

\fontfamily{Roboto-TLF}\fontseries{b}\fontsize{40pt}{40pt}\selectfont%
This is \f@series{} and should be b.

\fontfamily{Roboto-TLF}\fontseries{m}\fontsize{14pt}{14pt}\selectfont%
This is \f@series{} and should be m.

\end{document}
-----END cutting here may damage your screen surface-----

Running “pdflatex fontseries-bug.tex” and “mupdf fontseries-bug.pdf”
displays:

This is c and should be c.
This is bc and should be b.
This is m and should be m.

For some reason, \fontseries seems to be SOMETIMES cumulative.
This is directly against any documentation I can find online‽

Hi Thorsten,

Fontseries combines weight and width, and thus theoretically any
combination of weight (ul,el,l,sl,m,sb,b,eb,ub) and width
(uc,ec,c,sc,sx,x,ex,ux) are possible.

So c = width, b = weight, can be combined.
When you then switch to m = width this is reset.
See
https://tug.org/texinfohtml/latex2e.html#Low_002dlevel-font-commands
or
https://tex.stackexchange.com/questions/68745/possible-values-for-fontseries-and-fontshape

Closing the bug

All the best

Norbert

reopen 1003634
thanks

Hi Norbert,

I know that they can be combined.

This does not match the documentation.

This says:

   \fontseries{series}
          Select the font series. A series combines a weight and a width. Typically, a font supports only a
          few of the possible combinations. Some common combined series values include:

This implies that \fontseries{series} assigns series to \f@series.
Nowhere does it mention concatenation, or that ‘m’ (alone? others?)
resets it; the only mention of ‘m’ is…

          When forming the series string from the weight and width, drop the m that stands for medium
          weight or medium width, unless both weight and width are m, in which case use just one (‘m’).

… which basically says that, if your combination includes ‘m’ and
other things, you can drop one ‘m’.

I was there earlier! ;-)

The question, all answers and all comments also do *not* mention
that it’s cumulative and reset by ‘m’; they also indicate rather
the contrary (that it’s set to the value given as #1).

Respectfully disagree here. At least we have a severe documentation
bug, if not an implementation bug if the documentation is correct
in that it should not accumulate.

おやすみなさい,
//mirabilos

Norbert Preining dixit:

Yes, they *can* be combined.

If I do \fontseries{bc}.

But not if I do \fontseries{b} even if the previous
paragraph used \fontseries{c}.

At least not according to both intuitive understanding
and any kind of documentation I could find.

If you find whatever official documentation says different,
please do share… I couldn’t find that.

bye,
//mirabilos

Hi

I don't do ping-pong of bugs here, but this is not the least related to
Debian at all, and should be brought to the respective channels (TL
mailing list, LaTeX team tracker, IRC, ...).

All the best

Norbert

Hi Norbert,

thank you for forwarding it there then ☻

bye,
//mirabilos