Leave AP on
“A/T” is ‘arm AutoThrottle’ toggle, = throttle / spd autopilot (the left upper toggle on the AP shield = SHFT + R) // the visual is broken but in 747 a pink ‘autothrottle off’ warning show in the center screen //
you hand throttle up, so FLCH gets (bugged) proper power, and when it has that, you toggle A/T back on, which now works properly and brings you to to set Alt at speed.