Hello,
maintaining a consistent release version across all SEPMs and SEP 11 clients in an organization is not a requirement for support. It is supported to have newer SEPMs managing older SEP 11 clients, and older releases of the SEPM should be able to successfully manage SEP 11 clients that have a newer version of the software. These configurations are not recommended, but can arise as complex organizations perform testing and trials in accordance with their change management policies.
If the SEPM is 11 & clients 12. You will see the clients in SEPM, but the definitions will not be updated.
If the SEPM is 12 & clients are 11. no worries. It will be good