I agree that the old “most of the rubbish on Bali’s beaches comes from Java” claim is indeed a much used trope that can’t be supported simply by reading the labels on some of the items of rubbish.
I pay for a weekly recycling service from ecoBali Recycling so I did a quick citizen-science survey of all the plastic in this week’s bin and found that if I had dumped it on a beach then the packages would say that they came from - in no particular order - Macassar, Tangerang, Bekasi, Iowa (USA), Sidoarjo, Selangor (Malaysia), Bikaner (India), Singapore, Jakarta, Klaten, Thailand, Richmond (Canada) and Karawang with a single Bali manufactured product labelled as ‘Product of East Bali’.
There’s no doubt that some of the rubbish on Bali’s beaches does come from Java, and from much further afield as well, but the pictures on Sungai Watch’s website showing the thousands of tonnes of rubbish captured by their booms on Bali rivers speak for themselves as to where the majority comes from (see attached picture of what they say is a single day’s capture on a Bali river).