Raw SegWit TX look solid and verifies, but is rejected. Need help figuring out why
It's valid. In fact, pasting the transaction here actually shows a little checkmark next to the "signed" icon indicating a proper signature. 


It is not valid. That website did not validate anything. It just checked whether there was something in the scriptWitness field, and if there was, it was marked as signed. That checkmark does not mean that it is valid.

Your signature is simply incorrect. There's no other way to put it. Running it through my Bitcoin Core node gives

64: non-mandatory-script-verify-flag (Signature must be zero for failed CHECK(MULTI)SIG operation)


which is the typical invalid signature error.

Double check that you are creating the sighash correctly. Problems like this usually are the result of incorrectly serializing the transaction for signing.