So here is a redacted CLI-dump of the exact sequence I'm describing in my post. This is with untweaked keys and gpg 2.2.40 and a factory-reset yubikey.

So in case this was not clear... What I'm describing is very similar to the original description, but it is "inverted" - the untweaked key works flawlessly (import and decryption) except for keytocard. And the tweaked key can't be imported - either "Bad Secret Key" or asking for passphrase.

@onickolay Yes, I have. I have used --check-cv25519-bits and it said that it needs patching. I then did --fix-cv25519-bits and exported the key. Looking at the CV25519 private-key bytes produced by my code and by RNP, I confirmed that they did the exact same transformation.
When trying to re-import the exported key into gpg, I got the "Bad Secret Key" error again

Hi! I would like to chime in on this issue as I am having some weird problems with a CV25519 sub-key and after stumbling upon this thread, I think it is related to this.
Unfortunately, I can't post the key material here, because it is my actual encryption private-key.