Observations:
- TestKey1 (gpg 2.3) is an ECC-key (ed25519/cv25519) while TestKey3 (OpenKeyChain) is an RSA-key (rsa3072). I assume that OpenKeyChain supports ed25519/cv25519.
- TestKey1 (gpg 2.3) states that it supports some advanced OpenPGP features: features: 07 (= 0x04 + 0x02 + 0x01).
- TestKey3 (OpenKeyChain) states that it only supports one advanced OpenPGP feature: features: 01