When building LineageOS, how to use a Vendor Snapshot?

Question

I'm trying to build LineageOS and I need to use an older VNDK version (v30) for the vendor image because this is how the manufacturer does it and I need to be able to run a few of the manufacturer's vendor binaries for hardware support.

This is supported in AOSP with Vendor Snapshots. I've successfully built the v30 snapshot zip file according the documentation using the Lineage-18.1 branch. I then move to my Lineage-20 branch and install the vendor snapshot using the command in the documentation: python3 development/vendor_snapshot/update.py --local /path/to/snapshot.zip --install-dir vendor/VENDORNAME/vendor_snapshot 30

I then try to build my vendorimage with:

BOARD_VNDK_VERSION=30 mka vendorimage

But I just get errors that indicate it can't find dependencies in the vendor snapshot.

error: hardware/interfaces/power/aidl/default/apex/Android.bp:46:1: dependency "android.hardware.power-service.example" of "com.android.hardware.power" missing variant:
  os:android,image:vendor.30,arch:arm64_armv8-2a_cortex-a75
available variants:
  os:android,image:vendor.33,arch:arm64_armv8-2a_cortex-a75 (alias to os:android,image:vendor.33,arch:arm64_armv8-2a_cortex-a75,sdk:)
  os:android,image:vendor.33,arch:arm64_armv8-2a_cortex-a75,sdk:

I've confirmed android.hardware.power-service.example is in the snapshot.

I've done enough digging into this to realize that it's not an issue with just one or two dependencies, but instead it seems like the build system isn't picking up anything from the snapshot. I can see in strace that the vendor/VENDORNAME/vendor_snapshot/v30/arm64/Android.bp file is being read by the build system, but otherwise seems like the build is behaving as if the vendor snapshot wasn't installed at all.

Is there a step I have missed in installing the snapshot?

Footnote:
Here is how android.hardware.power-service.example appears in vendor/VENDORNAME/vendor_snapshot/v30/arm64/Android.bp:

vendor_snapshot_binary {
    arch: {
        arm64: {
            src: "arch-arm64-armv8-2a/binary/android.hardware.power-service.example",
        },
    },
    compile_multilib: "64",
    init_rc: [
        "configs/power-default.rc",
    ],
    name: "android.hardware.power-service.example",
    relative_install_path: "hw",
    shared_libs: [
        "libbase",
        "libbinder_ndk",
        "android.hardware.power-ndk_platform",
        "libc++",
        "libc",
        "libm",
        "libdl",
    ],
    target_arch: "arm64",
    vendor: true,
    version: "30",
    vintf_fragments: [
        "configs/power-default.xml",
    ],
}