So - there are a few different types of resources podman manages.
containers - These are instances of an image and the thing that “runs”. podman container ls
images - These are disk images (actually multiple but don’t worry about that) that are used to run a container. podman image ls
volumes - These are persistent storage that can be used between runs for containers since they are often ephemeral. podman volume ls
When you do a “prune” it only removes resources that aren’t in use. It could be that you have some container that references a volume that keeps it around. Maybe there’s a process that spins up and runs the container on a schedule, dunno. The above podman commands might help find a name of something that can be helpful.