Futures Demo: stop_market orders auto-cancelled ~53s after placement with no fill or error

Environment: Binance USD-M Futures Demo (https://demo.binance.com/)
API endpoint: POST /fapi/v1/order
Order type: STOP_MARKET
Observed: 2026-04-22

Issue:
Stop-market orders placed via the Futures Demo API are being
automatically cancelled approximately 53 seconds after placement,
with no fill, no error code, and no explanation in the order history.

Steps to reproduce:
1. Place a STOP_MARKET reduceOnly order on any USDT-M perpetual pair
(confirmed on AVAXUSDT, DOTUSDT, DOGEUSDT, AAVEUSDT)
2. Poll order status after ~60s
3. Status returns CANCELED with no fill and no reject reason

Expected: Order remains OPEN until stop price is triggered or manually cancelled
Actual: Order is silently CANCELED ~53 seconds after placement

Impact: Automated trading systems lose stop-loss protection unexpectedly,
triggering incorrect emergency position closes.

Sample order IDs from Demo session 2026-04-22:
- 1000000054696912 (AVAXUSDT, cancelled 53s after placement)
- 1000000054698315 (DOTUSDT, cancelled 53s after placement)
- 1000000054680514 (AAVEUSDT, cancelled ~55s after placement)

No repository file, test, or entry point is named. Start by reproducing the POST /fapi/v1/order flow against Binance USD-M Futures Demo and polling order status after 60 seconds; done requires identifying whether the connector contributes to the cancellation or documenting a confirmed upstream API behavior and actionable fix.

Written by the indexing model from the issue text.