I have no idea of the risk these days in Sumbawa, but if there still is the chance of malaria being present the risk would be higher in the wet season or few months after the wet season, while the risk latter in the year after months of no to little rain the risk would be lower, as less mosquitos.

I was about to say the whole malaria risk thing in Indonesia is pretty low these days as every year it gets better and better, but i just googled to see how many cases per year and although dropping they say there was still 1.3 million cases of malaria (not deaths) in 2018