Setting pdflatex output directory no longer automatically works

In an old project where I used a version of xsim prior to v0.18 I have a Makefile where I set the output directory of pdflatex:

latex = pdflatex --output-directory build

, and this used to work without any problems.

On my new machine I have:

pdflatex --version
pdfTeX 3.14159265-2.6-1.40.20 (TeX Live 2019)
kpathsea version 6.3.1
Copyright 2019 Han The Thanh (pdfTeX) et al.
There is NO warranty.  Redistribution of this software is
covered by the terms of both the pdfTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the pdfTeX source.
Primary author of pdfTeX: Han The Thanh (pdfTeX) et al.
Compiled with libpng 1.6.36; using libpng 1.6.36
Compiled with zlib 1.2.11; using zlib 1.2.11
Compiled with xpdf version 4.01

, and:

tlmgr info xsim
package:     xsim
category:    Package
shortdesc:   eXercise Sheets IMproved
longdesc:    This package helps in creating exercises and the corresponding solutions. It is the official successor of the exsheets package and fixes/improves various long-standing issues.
installed:   Yes
revision:    53893
sizes:       run: 273k
relocatable: No
cat-version: 0.18
cat-license: lppl1.3
cat-topics:  exercise exam expl3
cat-contact-repository: https://github.com/cgnieder/xsim/
cat-contact-bugs: https://github.com/cgnieder/xsim/issues
collection:  collection-latexextra

This does no longer works. Now each exercise only gets numbered but nothing else. Now I must add the build directory to the TEXINPUTS variable.

latex = TEXINPUTS=.:./build:${TEXINPUTS} pdflatex --output-directory build

Is this expected behaviour or a bug?

cgnieder / xsim

Setting pdflatex output directory no longer automatically works #60