Certainly the countries you mention do not have adequate, if any, trash disposal systems. So their trash, including plastic, gets dumped all over the place. Since they are coastal/island nations, a lot of that trash ends up in the ocean.
However, some of those countries have been importing huge amounts of plastic from the U.S., Canada, U.K., Australia and others, supposedly to recycle it, since it is not cost effective to do it here. I suspect that some percentage of their "recycling" consists of just dumping it.
Recently, China and Malaysia are no longer accepting foreign plastic for recycling. In fact, Malaysia is returning hundreds of tons of plastic to the countries of origin. We will see what happens then.