This issue will be fixed in the 2022.2 release, where the NoC compiler will protect the boot path and PMC-RAM by only allowing write requests that originate within the same SLR.
To this end, boot-path NIDBs will flag any incoming transactions so that the XMPU can reject write requests not originating within the same SLR.
The XMPU in the SLR will also be configured to allow reads only to specific PMC RAM space that should not contain security-critical information.