I do exactly what you describe - run multiple containers as various unprivileged service users simultaneously - and haven't had an issue doing it. The only thing that tripped me up was that I initially didn't realize that the service users needed to have a home directory created. I thought the service users wouldn't have a need for a home directory since they were single-purpose, but it turns out podman requires it to function, and if you try to run it as a user without one, it will fail to start in weird ways. Is there any chance that is the source of your issue?