Creating files in /tmp changes default group ownership? We are supposed to stop using /tmp?
I had the same experience when using /tmp but using $TMPDIR worked as expected:

$ touch "${TMPDIR}/tmp.txt"
$ ls -l "${TMPDIR}/tmp.txt"

-rw-r--r-- 1 luomat staff 0 Dec  3 08:46    /var/folders/8k/0bqq33mn439938qxp7j7c1fc0000gn/T//tmp.txt