Test with buildah-1.14.9-1.module+el8.2.1+6689+748e6520.x86_64 and it works as expect. So set this to verified.
Details:
$ buildah bud -t foo .
STEP 1: FROM centos

Getting image source signatures
Copying blob 8a29a15cefae done  
Copying config 470671670c done  
Writing manifest to image destination
Storing signatures
STEP 2: ENV foo=oo
STEP 3: ADD ./f${foo} /tmp/foo
STEP 4: COMMIT foo
Getting image source signatures
Copying blob 0683de282177 skipped: already exists  
Copying blob 8225927f5038 done  
Copying config 30af136a04 done  
Writing manifest to image destination
Storing signatures
--> 30af136a040
30af136a0407b864ef8a0487f07f2b9fecb16b523a5a2b1a887cc7baf0ae5b
$ podman run foo /bin/bash -c "cat /tmp/foo"
hello

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/RHSA-2020:3053