Rio Negro not getting rendered

Why is the Rio Negro multipolygon not getting rendered?

Because there are 16 reported issues with it!

You have the following:

For such big multipolygons I would suggest finishing the editing in Josm. It enables an overview of the entire mulitpolygon which you cant get in Potlatch, and using the validation tool will help pick out issues.

There are other tools for analysing relations (refer to the wiki), but in my experience Josm is the place to start here. Always use the relation dialog and sort the ways, this really helps to pin down where any gaps are.

Best of luck.

Looks like muralito made some fixes, but on OSMF servers the default map is re-rendered on low zoom levels (z0-z12) 1-2 times a month, so please be patient to see the results there.