As designed by Google? Yes. Does it break security in general? No. I am not deep into Android security, but AFAIK the main issue with custom ROMs is the open bootloader, which Graphene does not have.
I haven't seen any security problems unique to custom ROMs, especially if you relock the bootloader. The biggest issue is that you can lose device integrity or safetynet checks, which means you may not be able to use some apps, like banking.