As the autopilot had no other choice than to pitch violently down, to keep altitude, it has probably overdone a parameter in pitch and automatically disconnected itself with quite a pitch down.
Autopilot (A/P) Disengage Light
Illuminated (red) –
steady for any of following conditions:
ALT ACQ mode inhibited during A/P go–around if stabilizer not trimmed for single A/P operation
The NG doesn't do violent level offs during go around. If it is not trimmed properly, it will fly throught the MCP altitude rather than pitch down violently.
The indication for this is a steady autopilot disengage light.
There is also a Stab Out Of Trim light to tell you if the autopilot is not trimming correctly.
A normal go around on two engines is not a full thrust event. Nothing violent about it.
A wonky GS signal will do things to any aircraft.