Prasad is a weird exception, in that it’s controlled by Clans Burano and Ophris, who were Great Houses at one point but aren’t anymore. They have much more latitude than most satrapial administrations, because the Emprss recognizes she has trouble projecting force down there—and the last time she tried, the forces she projected went native and became, well, Clans Burano and Ophris. Sending some more legions down there to pacify Prasad and bring it back in line with normal satrapial standards runs the risk of those legions just doing the same damn thing.
They do continue to send tribute, because they don’t want to be conquered. The annual ship,ent is a big deal and a major operation, involving a lot of overland movement and brownwater sailing, and the route—actually two possible routes–has has economies all along the way reshaped by the needs of its passage.