I did a brief bench-test experiment, and by default ACRO mode provided an impulse response to counteract a disturbance. In my experiment rolling the flight controller quickly 90 degrees to the right and holding it there resulted in a short pulse on the aileron outputs to counteract the roll, then the servo output returned to center even though the flight controller was still rolled 90 degrees.
After enabling ACRO_LOCKING, rolling the flight controller quickly 90 degrees to the right resulted in the aileron outputs deflecting and remaining deflected.
So I think the docs on the ACRO mode need to be updated to make it clear that without ACRO_LOCKING there is no attempt to maintain attitude over longer periods of time.