Apple Intelligence unavailable when booting from an external SSD — is this a documented limitation?

I’m using a Mac mini (2024, M4) running macOS 27.2 in a Japanese-language environment, with Siri set to Japanese.

I cannot enable Apple Intelligence when booting macOS from an external SSD. However, I confirmed that Apple Intelligence works on the same Mac when booting from the internal SSD.

I contacted Apple Support, but the representative told me they could not find any internal information stating that Apple Intelligence is unsupported when booting from an external drive.

I also could not find this restriction in Apple’s published requirements:

Get started with the next generation of Apple Intelligence - Apple Support

A similar experience has been reported on Apple’s Developer Forums:

Security Verification

Has anyone else experienced this? Is there an officially supported way to use Apple Intelligence while booting from an external SSD?

If this is an intentional limitation, I would like Apple to clearly state it in the system requirements and support documentation. My internal SSD is only 256 GB, so moving my working environment back to it would be a significant inconvenience.