signrawtransactionwithwallet produces invalid Schnorr signatures

[apoelstra]

If you try modifying the wallet_taproot.py functional test like so:

diff --git a/test/functional/wallet_taproot.py b/test/functional/wallet_taproot.py
index 123a6a40893..f37fc906549 100755
--- a/test/functional/wallet_taproot.py
+++ b/test/functional/wallet_taproot.py
@@ -275,6 +275,12 @@ class WalletTaprootTest(BitcoinTestFramework):
             self.nodes[0].generatetoaddress(1, self.boring.getnewaddress())
             test_balance = int(self.rpc_online.getbalance()['bitcoin'] * 100000000)
             ret_amnt = random.randrange(100000, test_balance)
+            # Start...
+            rawret = self.rpc_online.createrawtransaction([], [{self.boring.getnewaddress(): Decimal(ret_amnt) / 100000000}])
+            rawret = self.rpc_online.fundrawtransaction(rawret, {"changePosition":1, "subtractFeeFromOutputs":[0]})
+            rawret = self.rpc_online.signrawtransactionwithwallet(rawret['hex'])
+            self.rpc_online.sendrawtransaction(rawret['hex'])
+            # ...end
             res = self.rpc_online.sendtoaddress(address=self.boring.getnewaddress(), amount=Decimal(ret_amnt) / 100000000, subtractfeefromamount=True)
             self.nodes[0].generatetoaddress(1, self.boring.getnewaddress())
             assert(self.rpc_online.gettransaction(res)["confirmations"] > 0)

You will get a failure of the form

test_framework.authproxy.JSONRPCException: non-mandatory-script-verify-flag (Invalid Schnorr signature) (-26)

on the sendrawtransaction line. You can verify by printing intermediate values that the output of signrawtransactionwithwallet shows complete: true and that the transaction indeed has something Schnorr-signature-shaped in its witness.