the question is this:

ECDH is done using a 257 bit (33 byte) pubkey, to derive a secret

if you just stick a 02 in front, then you are wrong 50% of the time when the actual pubkey is 03

i kinda mistakenly believed that BIP-340 solved this problem by just making all keys even but it seems i am wrong about that

gonna keep researching this, but the problem i'm having writing a test for ECDH is that the 32 byte key is missing the sign/oddness bit and thus deriving the correct decryption secret is a coinflip