That could also be explained by cultural differences, I don't think the evidence is overwhelming.

Or it could be discrimination. There are studies that show that black and white mixed races children did not score any higher than children with two black parents, while white people score higher than black people on average, which may point to some discrimination impacting IQ.

There are significant cultural and economic differences between those communities. These will be similar between Singapore and Malaysia. The Chinese and Indians are sufficiently recent immigrants for their culture to still have a lot in common (they speak the same language, follow the same religions, have inherited attitudes) with the places their ancestors came from.