Ah, fair enough. I have no specific knowledge of these types or this autopilot, but somehow the out-of-trim condition was missed, hence my questions.
I wonder if the aural alert can always be heard in a noisy cockpit and through headphones, and whether there is a trim light near the pilot's eye line.
Or maybe the pilot got confused and was moving the trim the wrong way - there are some clipped-on notes obscuring the trim wheel,