@kernel1983 I think there are two things to consider:
Curve25519/X25519 is not a cipher that can be used for public key signatures. Unless the "Curve25519" is a generalisation of the different expressions of the curve for ECDH in X25519 and EdDSA in Ed25519.
For Curve25519 there is a particular bit-mask applied to the (random) seed before it becomes a private key as it is used internally. The two are functionally identical as after application of the bit mask (before cryptographic operations) they are resulting in the same binary sequence.
I write a simple test to figure out the PrivateKey and SigningKey on version 1.5.0:
The output is always diffident for the first byte and sometimes the last byte:
May I know if I used this in the right way? Or is this a bug? I tried on both ubuntu 20.04 and macOS.