Propduffer.....
Or... the a/c "held" off the coast, further south of Banda Aceh.
It is the only way you can get "the fit".
And that means it wasn't a "constant" heading South.
And that doesn't fit the ATSB report *which wasn't written for "this" particular purpose anyway...but did state "a constant heading".
Edit (which is speculation) a "hold" would fit "an a/c in distress", rather than any other suggestion.
Although there are other possibilities, however unlikely they may be.