AA used to send their 737 to MAS for maintenance long ago (under TF), but then stopped it & now sending to Singapore I think.
Reason being is most probably that any aircraft sent to MAS enginineering is never released on time, while in Singapore it's always on time!
Even AAX which has A330 & A340 is not sending to MAS engineering, eventhough MAS has capability for A330 & A340 maintenance.