On Ubuntu-old (with cgroups v1 and runc; possibly with other weird settings), the very very first time rootless podman is invoked, it emits a level=error message (but succeeds with exit 0 regardless):
$ podman version
time="2021-05-06T13:13:04Z" level=error msg="overlay test mount with multiple lowers failed, but succeeded with a single lower"
Version: 3.2.0-dev
...
I can't reproduce on RHEL 8.4, possibly because I can't get a recent-enough version of podman. This looks like a very new error.
The message does not seem to happen after the first invocation of podman.