I seem to remember that the autopilot will continue to remain engaged with a reduction in speed (whether in say Alt Hold or in this case locked onto the glidepath) and continuing to trim the stabiliser for this new speed until the stall is approached when it stops trimming. No-one explained on the course whether the autopilot cuts out as a function of speed or due to the "out of trim situation
more probably because the autopilot coul not anymore follow the glide path, the A/C beeing fully trimmed.