Checked with  buildah-1.11.6-5.el8.x86_64 and it's fixed.


# date > foo
# cat foo
Sun Apr 12 21:30:45 EDT 2020
# rpm -q buildah
buildah-1.11.6-5.el8.x86_64
# buildah bud -t expand_env:latest
STEP 1: FROM centos
STEP 2: ENV foo=oo
STEP 3: ADD ./f${foo} /tmp/foo
STEP 4: COMMIT expand_env:latest
Getting image source signatures
Copying blob b05580fca2f9 skipped: already exists
Copying blob 9c0b77592056 done
Copying config 4d7220d50d done
Writing manifest to image destination
Storing signatures
4d7220d50dc9c8894699c066b55d6ab7fd36cfa1daa6679c8449df1ba3de5343
4d7220d50dc9c8894699c066b55d6ab7fd36cfa1daa6679c8449df1ba3de5343
# cat Dockerfile 
FROM centos
ENV foo=oo
ADD ./f${foo} /tmp/foo
# podman run localhost/expand_env:latest /bin/cat /tmp/foo
Sun Apr 12 21:30:45 EDT 2020

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:1482