Thanks very much, you are absolutely correct. I will mark the original answer correctly.
In essence, JWT should be structured with the correct header and claims as in the docs. The remainder were implementation details on my end that I recently cleared.
Jwt {
header: JwtHeader { typ: 'JWT', alg: 'ES256', kid: 'keyIDHere' },
body: JwtBody {
iss: 'issuer-id-from-AppStoreConnect-KeysSection',
aud: 'appstoreconnect-v1',
iat: 1630030166,
exp: 1630033766,
nonce: '90516455-0a3a-4a40-a863-eab63c7359ac',
bid: 'com.bundle.id',
},
signingKey: '-----BEGIN PRIVATE KEY-----\n' +
'YOUR\n' +
'PRIVATE\n' +
'KEY\n' +
'HERE\n' +
'-----END PRIVATE KEY-----'
}
