Import key material that is 64 bytes in size into a HMAC_SHA_256 KMS

Question

I've been given a 64 byte secret to be used with HMAC_SHA_256. I can readily use this secret to generate a hash in Java as per below

SecretKeySpec keyspec = new SecretKeySpec(secret, "HmacSHA256");
Mac sha256 = (Mac) Mac.getInstance("HmacSHA256").clone();
sha256.init(keyspec);
sha256.doFinal(prehash.getBytes())

I would like to outsource this logic to KMS, and import this 64 byte Key Material into KMS. However I get the InvalidCiphertextException error. Based on my research, it appears KMS only supports a 32 Byte key for HMAC_SHA_256.

However, the spec (and example Java code implementing the spec) supports arbitrary key sizes, it will simply sha256 anything larger than the block size of 64 bytes.

I want to understand why this restriction exists in KMS, unless I've missed something

For brevity, attempting to run the following aws kms commands

aws kms get-parameters-for-import \
    --key-id XXXXXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXX \
    --wrapping-algorithm RSAES_OAEP_SHA_256 \
    --wrapping-key-spec RSA_4096 > import.json

cat import.json | jq -r .PublicKey | openssl enc -d -base64 -A -out WrappingPublicKey.bin
cat import.json | jq -r .ImportToken | openssl enc -d -base64 -A -out ImportToken.bin

openssl rand -out PlaintextKeyMaterial.bin 64 #Note that 32 works, 64 does not

openssl pkeyutl \
    -encrypt \
    -in PlaintextKeyMaterial.bin \
    -out EncryptedKeyMaterial.bin \
    -inkey WrappingPublicKey.bin \
    -keyform DER \
    -pubin \
    -pkeyopt rsa_padding_mode:oaep \
    -pkeyopt rsa_oaep_md:sha256 \
    -pkeyopt rsa_mgf1_md:sha256


aws kms import-key-material --key-id XXXXXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXX \
    --encrypted-key-material fileb://EncryptedKeyMaterial.bin \
    --import-token fileb://ImportToken.bin \
    --expiration-model KEY_MATERIAL_DOES_NOT_EXPIRE