Month macros only recognised in first bib file

[moewew]

Consider the following MWE

\documentclass{article}
\usepackage{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@inproceedings{foo,
  author       = {Foo},
  title        = {Foo},
  booktitle    = {Foo},
  year         = {2017},
  month = jan,
}
\end{filecontents}

\addbibresource{biblatex-examples.bib}
\addbibresource{\jobname.bib}

\begin{document}
\cite{foo}
\printbibliography
\end{document}

With biblatex 3.9/Biber 2.9 I get the warning

Utils.pm:169> WARN - BibTeX subsystem: C:\Users\Moritz\AppData\Local\Temp\z858WUEh1k\gleaseflest.bib_2484.utf8, line 10, warning: undefined macro "jan"

And the month is not printed.

This happens only if the .bib file that uses the macro is not the first file loaded with \addbibresource. If the two \addbibresource lines are switched, there is no warning.

[logological]

Works fine with Biber 2.7, so the problem was probably introduced in 2.8 or 2.9.

[plk]

Fixed in 2.10 dev version. Will upload and plan bugfix release soon.

[plk]

Please try the 2.10 dev version - should be fixed

[moewew]

The MWE works for me now.
Thank you for the quick fix.

[ambs]

in 2020, and Mint still ships 2.9 :(

[moewew]

Yeah, Ubuntu's repository TeX live that you get from apt/apt-get is basically frozen at the point of release (or a while before that) and never updated after release, so especially with LTS versions your TeX live version may end up way behind current development. (Ubuntu 18.04 seems to be especially problematic because they ship Biber 2.9, but biblatex 3.10 ... https://bugs.launchpad.net/ubuntu/+source/biber/+bug/1770516).

The go-to way to obtain a more up-to-date system is to install a vanilla TeX live from TUG.org: https://tex.stackexchange.com/q/1092/35864