I was in Bali in June of 2016. I was looking for salak but hoped I might also find wani stragglers. It turned out the season ended about about two months prior. We discussed fruit with some of the hotel staff in Bali. When we were talking to some waitresses & asked about wani, they just about swooned -- obviously both were very fond of the fruit. One of these waitresses was from Java, either the Solo or Jogja area, don't recall which. She said they did not have wani on Java (or at least that part of Java?). M. caesia is well documented from Java so perhaps they just don't have the white fleshed wani form of caesia?
There are several forms of wani. I believe I posted a link to a paper earlier in the library section.
I visited Medan many years ago (probably about 30?). I don't recall a lot of fruit but it may have been when I visited. Be aware you are getting very close to Aceh province. Muslims in that area (Aceh) are especially devout so it may be a good idea to brush up on proper etiquette if visiting that area so as not to unintentionally offend anyone. People were pretty friendly there years ago.
John