steers the aircraft downwards with no indication or notification.
Have you flown a 737? There is the little issue of the circular-saw/trim-wheel whirring away beside your knee to let you know something is amiss.
the procedures to disable this "undisclosed" system behavior are documented somewhere deep within their manuals, but it seems they "forgot" to tell anybody about this
Isn’t the procedure to disable the system a memory item checklist, ‘Stab Trim Runaway’? Hardly a secret.