Mainnet order signing fails (invalid PublicKey / invalid signature)

[magnificent1208]

Steps
Get positions for account_index and confirm an open short (e.g. BNB market_id=25 short).
Send a reduce-only close order (market or IOC limit) signed by lighter-python.

Result
sendTx returns 21136 invalid PublicKey or 21120 invalid signature; position not closed.

What we tried
Verified mainnet connectivity and pulled positions via /api/v1/account (confirmed there is an open short to close).
Verified the registered API key via /api/v1/apikeys and used the non-zero api_key_index (DB pub_key matches server public_key).
Tried closing with reduce_only orders using different order types (market and IOC limit) and also tried cancel_order.
Tried both Python SDK flow and our internal backend flow that also signs and submits txs.
All attempts are rejected by mainnet with 21136 invalid PublicKey or 21120 invalid signature.