In current rear master code there is in
usr/share/rear/conf/default.conf

TMPDIR=""
export TMPDIR

That violates Unix standards e.g. see
https://en.wikipedia.org/wiki/TMPDIR
where the user can have TMPDIR set to what he likes
and then all Unix standard compliant programs would
store their temporary files (instead of the system default).

But for rear this does not work, see
#967 (comment)

rear does not honor a specified TMPDIR
instead it overwrites any specified TMPDIR
with an empty value by default.

@gdha
I also assingn you because according to
"git blame -w usr/share/rear/conf/default.conf"
you implemented that.

@gdha
is there a reasoning behind why rear does not work
in compliance with Unix standards regarding TMPDIR?

FYI:
When I comment out that in default.conf as

#TMPDIR=""
#export TMPDIR

then it just works for me as expected:

# unset TMPDIR
# rear -d -D mkrescue

is using build area '/tmp/rear.qT4gIvw0UUXUxnf'
and

# TMPDIR="/var/tmp" rear -d -D mkrescue

is using build area '/var/tmp/rear.gZvwTNixSS7uUyO'